Add tile server url option

Hi,
Here is a very simple patch that permits the choice of the tile server. It just add a "url" option to map.leaflet configuration and use it at TileLayer creation.

I think it's nice to offer this option to avoid the need of a cloudmade API key.
For exemple, I use the tile server url "http://{s}.tile.openstreetmap.fr/osmfr/{z}/{x}/{y}.png" and set the attribution to "Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://www.openstreetmap.org/copyright">ODbL</a>, Imagery © <a href="http://openstreetmap.fr">OSM_Fr</a>"