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.

This is the first chunk of #80. I'm happy to wait until #82 is in and rebase.
http://google.github.io/oauth2client/doc/oauth2client.client.GoogleCredentials-class.html
is the place it gets spit out and it puts the code block in a `<pre>` tag, which is a literal environment, so the tag does nothing (as you can see).

On re-build, the content is still in a `<pre>` environment.