Add filesystem cache

Add the new filesystem cache to salt. This only adds the files and startup/shutdown routines for the cache itself to master.py. Its not used anywhere yet. 

It also adds salt.caches to the package-build.

I put the cache in its own subdir instead of utils, because there might be more caches coming later (not necessarily from me :-) ). Feel free to move it anywhere you like if you think it fits better.