Move Dispatcher#{current,previous}Route initialization one level lower to avoid setting them before promise is resolved
Introduce `Dispatcher#_future_currentRoute` to keep route reference until `beforeAction` promise is fulfilled.

Should call action after resolving promise.
Variable naming change

@_future_previousRoute -> @nextCurrentRoute
@ _future_currentRoute -> @nextPreviousRoute
Merge pull request #719 from Sija/fix-beforeaction-promise

Fix Dispatcher#currentRoute being initialized too early