Add a uuid filter to add uuids to messages.

This is used to add a uuid to each message if it doesn't already have one, allowing
you to set the uuid as the document _id in Elasticsearch i.e.

"mappings": { "_default_": { "_id": {"path" => "@uuid"} } }

this allows you to send logstash messages through multiple paths in your network
(eventually being inserted into ES multiple times) without causing message duplication