omcache: more improvements, now passes initial tests

* cas support for get
* delta operations optionally return new value
* stats, noop, etc commands now use server_index instead of a dummy key for
  selecting the target backend
* map response error codes to omcache errors
* improvements to python bindings, support for py3k
* added omcache_pylibmc.py wrapper for compatibility testing
* various other fixes, especially for connection state handling