Panning the map updates latitude/longitude values. Fixes #44.

Listen for `center_changed` events and update `latitude` and `longitude` attributes. On `updateCenter` check if map center is already at the new location and if it ignore the request.

Fixes #44.
Is there a way to do something similar without wrapping it? I wrapped it in a Polymer element to test and show the two-way binding of `latitude` and `longitude` attributes. If it's impossible to listen for changes outside polymer context I can revert the demo changes.