Open id connect

@cvasilak I've added the method login to allow OpenID Connect login. would you mind reviewing?

If you want to test it with real app, see:
https://github.com/corinnekrych/aerogear-ios-cookbook-1/tree/AGIOS-287
with this realm config:
https://github.com/corinnekrych/aerogear-backend-cookbook-1/tree/AGIOS-287.SharedShoot.openIDConnect

Note: I haven't finish demo implementation yet. Need to talk to Andrea to make it nicer and display shoot images before submitting PR.
if ```userInfoEndpoint``` is not set, we should still invoke the  ```completionHandler``` block instead of failing silently