Add MessageIterator for pulling work off a pull queue.

Add a MessageIterator which pulls messages off a specified pull queue in
specified batch amounts. It exposes an iterator to go over the messages and
send out the payload from the messages
Remove get_messages call as the MessageIterator does what's needed.

get_messages is no longer needed.
Merge pull request #25 from beaulyddon-wf/message_iterator

Add MessageIterator for pulling work off a pull queue.