Add rabbitmq_policy custom type

Include support for RabbitMQ <3.2.0:
  * applyto field was not available before 3.2.0
  * syntax of the set_policy command changed in 3.2.0
If you look toward the end of the comments on #192, there was a question of whether there might be '@' in an exchange or queue name. I'm not sure if this applies to policy names as well. The proposed solution was to assume the last part of the name was the vhost and everything before the '@' was the name. We could replace split('@')[0] and split('@')[1] with rpartition('@').first and rpartition('@').last. Does that seem reasonable?