Add wrapper for buck so it can be executed easily

This downloads the same version of buck as "./go" does so
the two will play nicely together and share the same cache
(and only download buck once)

The buckw script takes the same arguments as buck itself.

buckw

@@ -0,0 +1,66 @@
+#!/usr/bin/env python2
+
+import hashlib
+import os
+import os.path
+import requests
+import shutil
+import stat
+import sys
+from subprocess import call
+
+buck_version = open(".buckversion").readline().rstrip()
+buck_hash = open(".buckhash").readline().rstrip()
+buck_pex = os.path.join("buck-out", "crazy-fun", buck_version, "buck.pex")
+
+url = "https://github.com/SeleniumHQ/buck/releases/download/buck-release-%s/buck.pex" % buck_version
+
+def download(url, out, hash):
+    base_name = os.path.dirname(out)
+    if not os.path.exists(base_name):
+        try:
+            os.makedirs(base_name)
+        except OSError as e:
+            if e.errno != errno.EEXIST:
+                raise
+
+    print "Downloading Buck. This may take some time"
+    response = requests.get(url, stream=True)
+    if response.status_code != 200:
+        raise IOError("Unable to download buck")
+
+    total_length = float(response.headers.get('content-length'))
+    count = 0
+    with open(out, "wb") as f:
+        md5 = hashlib.md5()
+        for chunk in response.iter_content(chunk_size=4096):
+            count += len(chunk)
+            done = int(50 * float(count) / total_length)
+            sys.stdout.write("\r[%s%s]" % ('=' * done, ' ' * (50-done)) )    
+            sys.stdout.flush()
+
+            f.write(chunk)
+            md5.update(chunk)
+
+        if md5.hexdigest() != hash:
+            raise IOError("Downloaded buck version doesn't match expected hash")
+
+if os.path.isfile(buck_pex):
+    md5 = hashlib.md5()
+    with open(buck_pex, "rb") as f:
+        for chunk in iter(lambda: f.read(4096), b""):
+            md5.update(chunk)
+    if md5.hexdigest() != buck_hash:
+        print "MD5 hashes don't match. Re-downloading"
+        download(url, buck_pex, buck_hash)
+else:
+    download(url, buck_pex, buck_hash)
+
+st = os.stat(buck_pex)
+os.chmod(buck_pex, st.st_mode | stat.S_IEXEC)
+
+sys.argv.pop(0)
+args = ["python2", buck_pex]
+args.extend(sys.argv)
+call(args)
+