Add SDCAlertViewTransitioning protocol

This protocol is created to make give other developers the freedom to decide how to show and dismiss alerts.
Fix build errors and warning in SDCAlertViewTransitioning.h
Make SDCAlertViewCoordinator default coordinator

Most developers will want to use the default SDCAlertView transitions, so use SDCAlertViewCoordinator as the transitionCoordinator if no custom coordinator was provided.
Get rid of SDCAlertView_Private.h