Get rid of Objective-C tests

They won’t be maintained, after all.
++Quick for Xcode 6 beta 3 support
Clean up memory management in Signal

Except for Signal.buffer, which is currently pretty #dangerzone.
Add pipeWithInitialValue for controllable Signals
Retain a promise's action until it's been resolved
Parameterize ScopedDisposable by innerDisposable

Since it layers memory management semantics on top of the inner
disposable, it’s important to be able to access the inner one without
holding on to a separate reference to it.
Stop observing notifications when Signal deallocs