add WMS support

Long time ago, I added a RMWMSSource to /route-me/route-me. This is the same thing but with some minor changes from my project + some changes to confirm with the mapbox fork.

 Example (from RMWMSSource.h):
 RMWMS *wms = [[RMWMS alloc] init];
 wms.urlPrefix = @"http://vmap0.tiles.osgeo.org/wms/vmap0";
 wms.layers = @"basic";
 RMWMSSource *wmsSource = [[RMWMSSource alloc] init];
 wmsSource.wms = wms;
 wmsSource.uniqueTilecacheKey = @"abc";
 [mapView setTileSource:wmsSource];