Small readability changes

Implemented class method in DataManager that takes care of
fetching summoners given a name, region, success&fail handler.
Makes code more readible in classes that actually make use of
the fetched summoners data.
(SignIn & SummonerVCs have been modified accordingly).

Fiddling with AppDelegate to see if app enters SummonerVC immediately
given an already registered user, but does not seem to do so.

TODO:
- SummonerManager does not seem to be saving summoners to core data
- Back button does not show for any summoners searched