Refactored TileUtil.js so that the tileset isn't hardcoded to be "Arbiter/osm".   Made it so that all tiles are within the root Arbiter/TileSets folder, openstreetmap tiles get cached to Arbiter/TileSets/OpenStreetMap/, and other layers get cached to Arbiter/TileSets/<server id>/<workspace prefix>/<featureType>