Add support for permalinks

This PR supersedes https://github.com/camptocamp/ngeo/pull/95. It adds two new services: `ngeoLocation` and `ngeoDebounce`. The former makes it possible to change the URL in the browser address bar. The latter makes it possible to debounce a function. Both services are now used in a `permalink` example, which shows how to change the URL in the browser address bar when the map view changes.

The example is available here: http://erilem.net/ngeo/location/permalink.html.