RACAggregatingSignalGenerator

A generalized, simpler, and less spooky version of the `RACQueuedSignalGenerator` proposed in #967 and #994. Part of resolving #992.

Hopefully the documentation is clear enough, but the basic idea is that invoking this generator will push a new signal onto `generatedSignals`. Consumers determine how to actually start those signals by how they join `generatedSignals`.

Effectively, this generator allows transformations to be performed over _all_ of the signals created by another generator. It's sorta like a generalization of `RACCommand.executionSignals`, except that the execution behavior isn't predetermined.

**To do:**

 - [x] Unit tests

@jonsterling