Added rabbitmq_policy support

Initial implementation of rabbitmq_policy.
Included sample usage as part of README. Works great in our production environment using a 2 clustered nodes on multiple exchange types too (in our case, including x-udp).
Usage sample:

```puppet
rabbitmq_policy { 'affiliate_queues':
    vhost      => '/',
    pattern    => '^affiliate_.*$',
    apply_to   => 'all',
    definition => '{"ha-mode":"all","ha-sync-mode":"automatic"}',
    priority   => 0,
}
```
@cyberious depends on your Ruby version IIRC.
I did tried that originally and it did work in my dev, but not in staging or prod. So I reverted to regex, since it was also conforming to used standards as of https://github.com/puppetlabs/puppetlabs-rabbitmq/blob/master/lib/puppet/provider/rabbitmq_user_permissions/rabbitmqctl.rb#L19