Add GoogleMaps v3.15 in externs
Include GoogleMaps v3.15 externs in build
Add google.maps.Geocoder in control

Now that GoogleMaps externs are included, a Geocoder could be added.
When pressing 'enter' or clicking the button, the googlemapsgeocoder
control now triggers the geocode method and populate the textbox
accordingly.

Work to do:
 * support multiple results
 * click on map to trigger geocode