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.
you can use a `<template is="auto-binding>`, but I actually don't think we need to add this in the demo. It's something the element should have been doing in the first palce.