Fix #215: Helper for int and double now work as expected.
Pinned protobuf at >= 2.5.0.

See discussion on Issue #215 for details.
Updated naming of global variable to follow PEP8.
Always cast int and long values to a long when preparing for a request.
Merge pull request #216 from jgeewax/master

Fix #215: Helper for int and double now work as expected.
Removing gcloud.credential.Credential class.

No instances of this class were ever created and it only had
one factory method, which is now a function. This class was
also misunderstood / misreferenced in the docstring mentions.