Can now use RetryProcessor with PhpAmqpLib

Fix #67 

I don't like so much this 2 different ways of using headers...

MessageProviders handle headers correctly (the always return a `headers` key in properties).
In this implementation we have to call the `addCustomHeaderToMessage` method when publishing.

Maybe I can avoid to change the actual implementation of the RetryProcessor and update only the `AmqpPhpLibMessagePublisher` which can change the `headers` key to `application_headers`.
It's a bit more "magical" but maybe it's more coherent.
What do you think ?
typo: extra ``x``