Fix return types for RACScheduler class methods

instancetype isn't really correct, since these methods return objects
of the same subclass no matter which class they're invoked against.
Fix outdated description for -afterDelay:schedule:
Implementation and tests for RACTestScheduler
Refactor other unit tests to use RACTestScheduler
CHANGELOG (needs update with PR link)
Add PR link into changelog
Merge pull request #716 from ReactiveCocoa/test-scheduler

RACTestScheduler