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.
I'd prefer if this wasn't wrapped in a Polymer element. We want to show people to use
it outside the context of Polymer and it's sugaring.