Refactor common sender responsibilities into RACDelegateProxy

This change relieves the users of RACDelegateProxy from performing the
following tasks:

1. Assigning the RACDelegateProxy to the delegator's delegate property.
2. Scoping selector signals to the lifetime of the delegator.
3. Unnecessarly specifying the `protocol` to request a selector's signal.
Add UIActionSheet+RACSignalSupport
Merge remote-tracking branch 'origin/signal-for-selector-from-protocol' into uiactionsheet-signal-support

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa.xcodeproj/project.pbxproj
Use unsafe_unretained for Mac compatibility
Move responsibility of assigning the delegate out of RACDelegateProxy
Use -reduceEach over map for clarity over tuple arguments
Test UIActionSheet with non-zero button index
Merge branch '2.0-development' into uiactionsheet-signal-support

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa/RACDelegateProxy.h
	ReactiveCocoaFramework/ReactiveCocoa/UITextView+RACSignalSupport.m
Descope RACDelegateProxy signals

The sender is now responsible for ensuring signals are scoped to the lifetime
of the delegating object, if so desired.
Remove no longer valid test

As of bf56916, RACDelegateProxy signals are not scoped to the delegator's
lifetime.
Copy documentation wording from UITextView+RACSignalSupport.h
Publicize UIActionSheet+RACSignalSupport.h
Remove managing delegate assignment from RACDelegateProxy
Merge pull request #611 from ReactiveCocoa/uiactionsheet-signal-support

Add -[UIActionSheet rac_buttonClickedSignal]