Added missing action sheet to choose between multiple twitter accounts

Hey guys! I came across your code and found out that you were missing showing an action sheet when multiple twitter accounts were configured on device, so that the user is able to pick one of them to authenticate.

I just added the necessary code in AITwitterAuth.m, and needed to add some category on UIActionSheet to handle callbacks via blocks instead of delegates (as that part was done inside a class method).

I have no idea why the pod-related files did mess up, all I did was run "pod install" in order to be able to compile.

Cheers!