fix($transition) get a single transition to run.

- add Path.reverse to reverse a path inline (like Array.reverse)
- add "State X is already defined" to StateQueueManager.register() and flush()
- hook up Transition.run() in $state.transitionTo
- copied state.to/state.from to toState/fromState.  meh
- fix bug in entering-states code
- init _fromPath in $transition.init()
- fix transitionSpec to not try to clobber the root state ('')