Make ObservableProperty changes thread-safe

It was already mostly there, since it has to serialize on a private
queue anyways. Just wrap mutations with that queue too.
Allow reading ObservableProperty concurrently
Improve Action/ObservableProperty debugDescription
Refactor Action to simplify, not use a scheduler
Rename Action.execute to apply

This better indicates its deferred nature, and is reminiscent of
“partial application.”
Fix accidentally-recursive Action initializer
Fix out-of-date docs for `value` and `errors`
Merge remote-tracking branch 'origin/swift-development' into improve-actions

Conflicts:
	ReactiveCocoa/Swift/Action.swift
Add a CocoaAction wrapper class, for binding to UI
Test KVO compliance for enabled/executing
Add missing optional to CocoaAction.execute param
Merge pull request #1628 from ReactiveCocoa/improve-actions

Improve Action class, add CocoaAction wrapper, remove all automatic scheduling