first pass - extending multiprovider to handle requests that have a "+" in the path (seperating individual layers for ex: /buildings+places+pois/123/132.json) - requires the config file to have a empty layer called "+" with class "TileStache.Goodies.VecTiles:MultiProvider" and empty names [].
using comma-delimited list instead of +
isNotValidLayer -> isValidLayer
global _delimiter set to comma
not dependendent on the config file anymore
camelCase to snake_case
Merge branch 'extend-multiprovider' into integration-1

Conflicts:
	TileStache/__init__.py