Add RACAggregatingSignalGenerator
Move RACSignalGenerator+Operations into main header

There will be few signal generator operators that aren’t coupled to a
specific subclass, so it’ll just be easier to define them in the
headers for those subclasses.
Replace generator initializers with class constructors

Purely an aesthetic choice, but class constructors look better than
+alloc/-init… when put into a chain of operations.

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa/RACQueuedSignalGenerator.h
	ReactiveCocoaFramework/ReactiveCocoa/RACQueuedSignalGenerator.m
	ReactiveCocoaFramework/ReactiveCocoa/RACSignalGenerator+Operations.m
	ReactiveCocoaFramework/ReactiveCocoaTests/RACQueuedSignalGeneratorSpec.m
Hopefully better phrasing for -postcompose:

h/t @jonsterling
Merge branch 'empty-subscription' into aggregating-generator
Actually implement -aggregate
Unit tests for RACAggregatingSignalGenerator
Merge remote-tracking branch 'origin/3.0-development' into aggregating-generator

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa.xcodeproj/project.pbxproj
	ReactiveCocoaFramework/ReactiveCocoa/RACSignalGenerator+Operations.m
	ReactiveCocoaFramework/ReactiveCocoa/ReactiveCocoa.h
Merge pull request #997 from ReactiveCocoa/aggregating-generator

RACAggregatingSignalGenerator