Assert that the result of a -flattenMap: block isn't nil
Merge branch 'master' into nil-bound-streams
Test for a stream generically (not just a signal)
Merge pull request #941 from ReactiveCocoa/nil-bound-streams

Assert that the result of a -flattenMap: block isn't nil
Promotes UICollectionViewCell category to UICollectionReusableView

This allows use of rac_prepareForReuseSignal on supplementary views.
Bump for jspahrsummers/libextobjc#51

Fixes #952.
Merge pull request #953 from derrh/master

Promotes UICollectionViewCell category to UICollectionReusableView
Merge branch 'master' into 3.0-development

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa.xcodeproj/project.pbxproj
	ReactiveCocoaFramework/ReactiveCocoa/ReactiveCocoa.h
	ReactiveCocoaFramework/ReactiveCocoa/UICollectionReusableView+RACSignalSupport.h
	ReactiveCocoaFramework/ReactiveCocoa/UICollectionReusableView+RACSignalSupport.m
	ReactiveCocoaFramework/ReactiveCocoa/UICollectionViewCell+RACSignalSupport.h
	ReactiveCocoaFramework/ReactiveCocoa/UICollectionViewCell+RACSignalSupport.m
	ReactiveCocoaFramework/ReactiveCocoa/UICollectionViewCell+RACSupport.h
	ReactiveCocoaFramework/ReactiveCocoa/UICollectionViewCell+RACSupport.m
	ReactiveCocoaFramework/ReactiveCocoaTests/UIKit/UIKitTests/UICollectionReusableViewRACSupportSpec.m