Pull the UTF grid response handling into the server and add caching

[prashtx]

Right now we kick a response object over to nodetiles-core and let it do everything. Let's use nodetiles-core as the renderer, and then we can implement cache control responses, caching, etc. in the server code.

[prashtx]

Leaflet uses the same callback function for the tile.json JSONP requests. That means we can use the ETag cache and the S3 cache, like with the tile images.