Fixes to OAuth2WebClient and project restructuring

With these fixes the following tests passes:
* oauth2client.py
* oauth2client_clientsecrets.py
* oauth2client_util.py
* oauth2client_xsrfutil.py

The other oauth2client tests have not been fixed since they require additional packages and set-ups.
I also did some small manual tests with real data to verify.

On top of that the package can now be installed using this command:
```
pip install git+https://github.com/enorvelle/GoogleApiPython3x.git
```