fix for issue #45

add wms tilecache scheme (z/000/000/x/000/000/y.png)
fix for issue #45 second attempt

referencing getKey method from:
http://svn.osgeo.org/tilecache/trunk/tilecache/TileCache/Caches/Disk.py
Merge pull request #46 from MichaelMarkieta/master

fix for issue #45