Add ol.Geolocation

A w3c geolocation object. The object _exports_ the position of the device (a projected `ol.Coordinate` point) and the position accuracy in meters.

Example usage: bind the center of the map to the device position:
```javascript
...
var view = map.getView();
var geolocation = new ol.Geolocation(view.getProjection());
view.bindTo('center', geolocation, 'position');
```