mysql wsrep stats

I've added option to gather some stats specific to MySQL Galera replication setup ( more details here http://www.percona.com/doc/percona-xtradb-cluster/5.5/wsrep-status-index.html )

as per https://github.com/collectd/collectd/pull/641 I used generic datatypes and added one, window_size as there wasn't any existing one that would fit. Also rebased on current master instead of latest stable