Use the $parse getter when evaluating expression string

Allows for expressions other than the name of the event handler function - needed when using controllerAs:

```
<div ng-controller="hmCtrl as ctrl" hm-tap="ctrl.onHammer"></div>
```

Also, removed the $scope.$apply call from the demo code, since the expression evaluation code will always call $scope.$apply when needed. Updating a scope property in the dragging demo to show x and y coordinates, to show that UI is being updated without explicitly calling $scope.$apply.