If there is no headers, just return empty HashMap (otherwise, it will crash with NPE)
Allow ConnectionConfig to use either URI or named parameters.
Merge pull request #11 from Dictanova/npe_headers

RabbitMQMessageScheme: Don't crash if headers is null
Merge pull request #12 from Dictanova/connection_with_uri

Allow ConnectionConfig to use either URI or named parameters.
emit every single message currently queued locally in QueueingConsumer on a single nextTuple call on the spout
fix null pointer exception when rabbitmq.uri is not provided