FirebaseUI 0.3.1 Release

Major changes to the Cocoapod structure:

1. Introduction of /Core and /Auth subspecs
2. Switched out strings for enums in the auth providers (see FirebaseAuthConstants.h/m)
3. Added a completion callback for FirebaseLoginViewController to handle captive portal login scenerios

Note that these changes (particularly 2) are backwards incompatible with 0.3.0--since we're below 1.0 we're a little looser than we'd like to be.