Add comparison with 'bmemcached'

[raff]

Just found this other memcached pure python client that may be worth mentioning in the list of other clients:
https://github.com/jaysonsantos/python-binary-memcached

It implements the binary protocol and support a "cluster" of server (but it simply write to all servers on set and does a round-robin / stop at first result on get)

[harlowja]

Or other idea is that this client also gets a binary protocol impl, I don't know about license compat but https://github.com/jaysonsantos/python-binary-memcached/blob/master/bmemcached/protocol.py seems like it does just that.

[jogo]

Related to Issue #54