mongodb_backend - Inclusion of optional memcached caching

Support for optional memcached caching is implemented, providing for a
substantial improvement when remote mongodb servers are used. Support
degrades gracefully for both missing the python-memcache module or not
having an active memcache server. There is a default memcache item
decay time of 5 minutes, but that can be set separately. Use of memcached 
requires defining the host and port, default behaviour is the current 
non-cached behaviour.