Stub ActionSpec
Initialize Actions with scheduler to serialize on
Test and fix enabled/executing default behavior
WIP more enabled/disabled tests for Action
Merge branch 'swift-development' into action-tests
Improve ColdSignal and HotSignal debugDescription
Back `enabled` with an ObservableProperty

This avoids issues around duplicating side effects, like missing values
on `enabledIf`.
Test Action.execute
Merge pull request #1614 from ReactiveCocoa/action-tests

Action tests