Switched Memcache cache to use pylibmc

This PR replaces the dependency on python-memcached with [pylibmc](https://pypi.python.org/pypi/pylibmc). pylibmc seems a bit more well documented and might have a few more features, for example connection pooling. It also supports SASL, making it easy to use with for example Heroku (it is the library that is recommended for using Python and Memcache on Heroku).

Also adds functions to set username and password, as well as controlling cache pool size.