Remove RACAggregatingSignalGenerator

RACSamplingSignalGenerator and RACAction will subsume its behaviors.
Ensure -not always returns BOOL-typed NSNumbers
Add executionSignals and results signals to RACAction

Also cleaned up the threading, and removed KVO for simplicity and
performance.
Turn RACAction into a RACSignalGenerator
Additional tests for new RACAction features
Simplify RACAction generation code
Update CHANGELOG