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.
Good question. The examples hosted on github.io (using the `gh-pages` make target) are not compiled. So we cannot use goog in the ngeo examples. We have to use exported symbols/properties only. That is the reason I've added "exports" for the `ngeoLocation` service. These exports are not necessary/used when you compile your app code with ngeo.

I will add a note in the README about that. Thanks for asking!