Serve TileJSON

Adds a "tile.json" resource to the root of the simple built-in debugging HTTP server. The TileJSON is built from the `mapnik::Map` object that's being served, so should be up-to-date with the tiles being created.
Nope, that's why there's a TODO in there to replace this with something that isn't super-hacky :smile:

If we merge this, then I'll add an issue+story for replacing this properly.