Add `default_port` option for compatibility with libmemcached.

libmemcached ignores the default memcached port when hashing via ketama_weighted:
http://bazaar.launchpad.net/~tangent-trunk/libmemcached/1.2/view/head:/libmemcached/hosts.cc#L293

To match the behaviour, use `new HashRing(hosts, 'md5', {default_port:11211})`
Merge pull request #23 from rcoup/libmemcached-compat

Add `default_port` option for compatibility with libmemcached.