Adding authenticator

This Pull request is adding an account manager for the github account.
The account manager is structured in 3 layers, BetterAccountAuthenticator is a cleaned up version of the account manager aimed at simplifying the interface. Reactive authenticator is an even simpler interface cleaned using RxJava on top of the previous layer. And finally the Gertherb layer is the implementation for our project.
Not that bad if it is trough the android account manager system since it is properly encrypted but yes best option is a proper refresh logic. It all depends on what the Github api allows.