Bump version to 0.5.5.
Add PendingBalancedRequester.

This requester is a special load balanced requester which keeps track of open, pending requests for each Responder this Requester knows. By making sure each new request goes to the Responder with the minimum open requests, a very efficient load balancing scheme is achieved.
Bump version to 0.5.6.
Assign a uuid to each socket in PendingBalancedRequester. Send method returns the uuid of the socket after a successful send operation, so users can keep track of which socket each request went.
Bump version to 0.5.7.
Add benchmark tests for publisher and subscriber