allow serving static files from the tiles/ directory

if static files (and sub-directories) are present, don't route through tileserver.php
this allows:
1. to switch from a static tree to an .mbtiles, and back, without changing URLs
2. to roughly build a cache for (a subset of) the tiles
Merge pull request #3 from Fil/allow-static-files

allow serving static files from the tiles/ directory