Add -[UIActionSheet rac_buttonClickedSignal]

Depends on #608 

This adds `-rac_buttonClickedSignal`, a convenience method on `UIActionSheet`, which returns a signal that sends the index of button clicks.

This includes changes to `RACDelegateProxy`, with the purpose being to move responsibility previously held by the caller into `RACDelegateProxy` (66cd6b31d01f5aea307bb334746ff13873926a3b). The main two responsibilities are:

1. Managing the delegate property.
2. Scoping the selector signals to the lifetime of the delegator.