fix($state.transitionTo): trigger $stateChangeCancel appropriately

Current behaviour:

We are in state A. We start a transition to state B. `$stateChangeStart`
is triggered. Before `$stateChangeSuccess` is triggered for state B, we
start a transition back to state A. No state events are triggered and we
are left hanging without any notification.

Updated behaviour:

When the state change to B is superseded by the state change back to A,
`$stateChangeCancel` is broadcasted on `$rootScope` for the transition from
B -> A. This behaviour makes sure that for every `$stateChangeStart` there
is a corresponding `$stateChange<Success|Error|Cancel>` thus completing the
lifecycle.

Fixes issue angular-ui#3027

Author: zeusdeux