Adds Google Oauth2 authentication

[pallan]

This is a WIP branch to add Google Oauth2 integration to the application. It attempts to mimic the manner in which GitHub is integrated and as a result causes some duplication of code. In its current state Google Oauth does work once you have the client key and secret configured.

I created this PR to try to get feedback and some help to finish this off

[TeatroIO]

I've prepared a stage. Click to open.

[coveralls]

Coverage decreased (-0.3%) when pulling d15ca255420bca76ea1881b0e2e4bb74e23a8d39 on pallan:omniauth into d50a69c on errbit:master.

[stevecrozz-]

Nice idea. Is your intention to automatically create an errbit account for anyone who has a google account? I wouldn't want that, but I'm interested in your particular use case. Maybe you do want that.

[pallan]

Our use case is that we use "Google Apps for Work" (or Business or whatever the new name is) to manage our organization. We use the OAuth provided by it to auth against a lot of our systems. This allows us to do the same thing with errbit.

As such, there is probably a couple more config variables that can be thrown in to enforce restricting it to our organization.

[arthurnn]

Is this good to be merged? @pallan anything else you need here?

[pallan]

It looks like I will have to resolve some merge conflicts. Plus I think there are a couple of missing tests that I wanted to address. I will try to have that all ready for you next week.