Fix crs to work with proj4 leaflet

Great library, I've got a project that needs a background map in a different co-ordinate system. proj4leaflet plugin solves this but I couldn't use it with your library as it require crs to be defined as a special object.

I changed the getMapDefaults function to check if crs is an object first if not it will work as normal.

                 if (isObject(userDefaults.crs)) {
                     newDefaults.crs = userDefaults.crs;
                 } else if (isDefined(L.CRS[userDefaults.crs])) {
                      newDefaults.crs = L.CRS[userDefaults.crs];
                  }

I also provided an example wms-with-different-projection example and added bower dev dependencies to proj4 and proj4 leaflet.

Hope you think the changes are OK.

Cheers for developing a great directive

Tom