This replaces the old functionality of ensuring no files or directories have subscriptions in javascript. We allow this now, but we just don't allow leaked watches at the native layer.
Path watching resumes once the file is saved again. This commit allows files to be created for as-yet nonexistent paths. We won't call `$native.watchPath` until we have at least 1 subscription to the file in JS and the file exists on disk.
Also, we moved execution of the path watcher callbacks until after the callbacks data structure is updated in order to avoid confusing behavior in specs.
