feat($cordovaGeolocation): watchPosition promise

```javascript
var watch_geo = $cordovaGeolocation.watchPosition();

watch_geo.then(angular.noop, failFunc, function(location) {
  console.log(location);
  sendBeacon(location);
});

watch_geo.cancel();
// or
watch_geo.clearWatch();
```
(The docs also need to be changed. I'm editing this PR github but I can update it in a bit)