add support for tile layers based on GeoWebCache tilestorage

The tilestorage can be served from a normal http web server like apache or nginx. This makes it possible to deploy prerenderred tiles using a normal apache/nginx server, no need to deploy GeoWebCache+Java+Tomcat on a production server.

The retrieved URL is built up like '{url}/path/to/cache/EPSG_900913_{zoom}/{dir_x}_{dir_y}/{x}_{y}.png', e.g. "http://myserver.org/geowebcache_data/the_gwc_layer/EPSG_900913_00/1_1/03_02.png"