Make RabbitMQMessgeScheme work on remote clusters.

Lack of implements serialized on envelope and 
properties classes meant it would work on a 
local cluster but not a remote one. When used
on a remote cluster you would get a:

java.io.NotSerializableException: 
io.latent.storm.rabbitmq.RabbitMQMessageScheme$Envelope
Merge pull request #7 from SeanTAllen/master

Make RabbitMQMessgeScheme work on remote clusters.