Added a python helper for check_header.sh in git pre-commit script.

On my box this speeds up the header check from 5 seconds to a little over 1 second.

```
 build-support/bin/check_header.sh        | 46 ++--------------------------------------------
 build-support/bin/check_header_helper.py | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 72 insertions(+), 44 deletions(-)
```

Testing Done:
```
~/Src/Pants zundel/cheader * git commit -a
Checking packages
Checking imports
Checking headers
ERROR: All .py files other than __init__.py should start with the following header:

# coding=utf-8
# Copyright YYYY Pants project contributors (see CONTRIBUTORS.md).
# Licensed under the Apache License, Version 2.0 (see LICENSE).

from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
                        unicode_literals, with_statement)

---
The following 1 file(s) do not conform:
  src/grody.py
```

Bugs closed: 1250

Reviewed at https://rbcommons.com/s/twitter/r/1910/

build-support/bin/check_header.sh

@@ -1,50 +1,8 @@
 #!/usr/bin/env bash
 
-set -euo pipefail
-IFS=$'\n\t'
+set -e
 
 REPO_ROOT="$(git rev-parse --show-toplevel)"
 cd ${REPO_ROOT}
 
-expected_header=$(cat <<EOF
-# coding=utf-8
-# Copyright YYYY Pants project contributors (see CONTRIBUTORS.md).
-# Licensed under the Apache License, Version 2.0 (see LICENSE).
-
-from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
-                        unicode_literals, with_statement)
-
-EOF
-)
-
-py_files=$(find src tests pants-plugins examples contrib -name '*.py' -not -name __init__.py)
-known_bad_headers=$(cat <<EOF
-tests/python/pants_test/tasks/false.py
-tests/python/pants_test/tasks/true.py
-EOF
-)
-
-bad_files=""
-for file in $py_files
-do
-  if echo "$known_bad_headers" | grep -Fx "$file" >> /dev/null
-  then
-    #known bad file, skip
-    continue
-  fi
-  header=$(head -7 "$file"|sed -e 's/20[0-9][0-9]/YYYY/' -e 's/	/        /g')
-  if [[ "$header" != "$expected_header" ]]
-  then
-    bad_files="${bad_files}"$'\n'"${file}"
-  fi
-done
-
-if [[ "x$bad_files" != "x" ]]
-then
-  echo "ERROR: All .py files other than __init__.py should start with the following header"
-  echo "$expected_header"
-  echo "---"
-  echo "The following files don't:"
-  echo "$bad_files"
-  exit 1
-fi
+build-support/bin/check_header_helper.py src tests pants-plugins examples contrib

build-support/bin/check_header_helper.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python2.7
+#
+# Helper for check_headers.sh to check .py files in the repo to see if they start with the
+# appropriate headers.
+#
+# usage: check_header_helper.py dir1 [ dir2 [ ... ] ]
+
+import os
+import re
+import sys
+
+
+EXPECTED_HEADER="""# coding=utf-8
+# Copyright YYYY Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+"""
+
+
+def check_header(filename):
+  """Returns True if header check passes."""
+  try:
+    with open(filename, 'r') as pyfile:
+      buf = ""
+      for lineno in range(1,8):
+        line = pyfile.readline()
+        # Skip shebang line
+        if lineno == 1 and line.startswith('#!'):
+          line = pyfile.readline()
+        # Don't care much about the actual year, just that its there
+        if line.startswith("# Copyright"):
+          year = line[12:-4]
+          if not re.match(r'20\d\d', year):
+            return False
+          line = "# Copyright YYYY" + line[16:]
+        buf += line
+      return buf == EXPECTED_HEADER
+  except IOError:
+    return False
+
+def check_dir(directory):
+  """Returns list of files that fail the check."""
+  failed_files = []
+  for root, dirs, files in os.walk(directory):
+    for f in files:
+      if f.endswith('.py') and os.path.basename(f) != '__init__.py':
+        filename = os.path.join(root, f)
+        if not check_header(filename):
+          failed_files.append(filename)
+  return failed_files
+
+
+def main():
+  dirs = sys.argv
+  failed_files = []
+  for directory in dirs:
+    failed_files.extend(check_dir(directory))
+  if failed_files:
+    print 'ERROR: All .py files other than __init__.py should start with the following header:'
+    print
+    print EXPECTED_HEADER
+    print '---'
+    print 'The following {} file(s) do not conform:'.format(len(failed_files))
+    print '  {}'.format('\n  '.join(failed_files))
+    sys.exit(1)
+
+if __name__ == '__main__':
+  main()