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)