feat(ngEnter) Create directive to exec function on 'enter' keypress
feat(EventService) Add support for promises in read() method

This way we can return the promise directly to a controller's scope like this:

```javascript
$scope.event = EventService.read(eventId);
```

Instead of having to always use a callback:

```javascript
EventService.read(eventId, function(data) {
    $timeout(function() {
        $scope.event = data;
    });
});
```

Of course the callback version is still useful for some use cases, so we
leave the option there in case it's needed.
feat(AppCtrl) Add $route option to configure visibility of nav panels

If it's not defined in the $route definition, the panels are shown by default.
The `style="display: none"` is used to initially hide the panels so we don't
get a flicker before the HTML is compiled.
fix(FirebaseCollection) Execute metafunction on 'child_moved' event
feat(UserService) Add exists() method to check if user exists