Adding layer control in leaflet for switching layers

Quick and dirty change, but could be usefull (it is for me at least)
![layercontrol_screenshot](https://cloud.githubusercontent.com/assets/807703/2802473/c545c30a-cc8e-11e3-8dc5-1a6a95c53aa7.png)
Should (partially) fix #5 , still needing to make it optionnal.