[PATCH] Mulitple layers adding errors + leaflet layers and map_id on the map object

As written in the manual, when we have multiple base layers on a map:

http://leafletjs.com/examples/layers-control.html

Also note that when using multiple base layers, only one of them should be added to the map at instantiation, but all of them should be present in the base layers object when creating the layers control.

Attached a patch to fix this issue + the instance of layers + map_id in order to simplify map modifications triggering "leaflet.map".