Added AMQP suppoert and support for the subscribers to decide if they want to defer an event

I've done four things here. 

### Added AMQP support
A service that talks to a AMQP queue. Added some config to that service as well. 

### Fixed support for subscribers to decide if they want to defer an event
If a listener listens to ```fervo_deferred_event.listener``` then will the event be deferred for that particular listener. This was a minor fix with a DummyEventDisatcher. The issue was that you cannot serialize or unserialize PDO instances. 

### Added a Message
The string that you publish on a Message queue should be on some standard form. I choose to use a HTTP-like protocol. Symfony may send some parameters to the worker like fastcgi_host and path. With these settings you could run multiple Symfony applications with the same message queue and worker. 

### Extended the documentation
Since this is quite a PR I've written lots of docs. Both PHP-doc and in the Readme.