Make RACAble macros slightly easier to read
Added a separate method to observe with an initial value

Also fixed keyPath copying and added some parameter assertions.
Use -rac_signalWithStartingValue… for RACAbleWithStart()
Unit test
Merge pull request #442 from ReactiveCocoa/defer-kvo-start

Only read RACAbleWithStart's initial value upon subscription
include a test configuration
++specta