Add watchCurrentPosition to GeolocationService

Added the watchCurrentPosition using a deferred's `notify` method.

Usage:

```
GeolocationService.watchCurrentPosition()
   .then(null,null,function (geolocationData) {
       var position = geolocationData.position;
       
       if (!withinBounds(position)) {
           GeolocationService.stopWatcher();
       }
   });
```