Add Subscriber and IronmqSubscriber

- Add support for receiving pipeline messages in a Rails app
- Add IronMQ push queue subscriber endpoint via Sinatra
- Add script for setting up endpoints as subscribers

TODO: 

- [x] Emit the event type in the envelope {CREATED/UPDATED/DELETED}
- [x] Add default handler {logger/CRUD}
- [ ] Add key in envelope and accept a comma separated list of keys in the subscriber
- [ ] GPG encrypt when logging
- [ ] Update README with documentation
Apply same logic than for api keys (comma separated list, to handle easy rotation)