Add Google as an authentication option.

Requires GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET to be present in the
.env file.
Update readme to include Google authentication config.

Refactored the config section of the readme to make it a bit more
obvious how to generate the OAuth tokens and which parts are optional /
mandatory.
Swap order of Google and Zendesk authentication options.
Correct line spacing at the end of omniauth file.

Applying coding convention fix from pull request.
Update readme to make general app env descriptions more accurate.
Merge pull request #8 from tonymilne/master

Add Google as an authentication option.