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,
}
```
This has been tested in ruby 1.8.7, 1.9.3 and 2.0.0, please let me know what version are you running against that you are not seeing this behavior?  And just because it is other code does not mean it is clean code, if there is a simpler solution that is more readable we should be utilizing it.