Removing docs from code repository.

Docs are now checked in to the gh-pages branch and
serve via:
google.github.io/oauth2client
Updating makefile and gitignore for building docs.
Merge pull request #82 from dhermes/remove-docs

Removing docs from code repository.
Refactoring _get_environment.

- Removes use of "global" by putting the env name in another
  namespace.
- Splits logic into digestible pieces.
- Also makes slight change to GoogleCredentials docstring.