Add support for reconfig cluster membership operation

[hannosch]

ZK 3.5 introduced a new reconfig operation to change the cluster memberships at runtime. The feature was tracked in https://issues.apache.org/jira/browse/ZOOKEEPER-107. The bulk of the work happened in http://svn.apache.org/viewvc?view=revision&revision=1453693.

This is the new type code 16.

[rgs1]

WIP: https://github.com/rgs1/kazoo/tree/support-reconfig, comments welcomed! Will add tests later this week.

[rgs1]

So this PR has basic coverage for this feature: #333. That being said, there are some server-side bugs to be addressed (i.e.: https://issues.apache.org/jira/browse/ZOOKEEPER-2202), but it would be nice to have this in so I can support it in zk-shell!