Tail for Queues

Given a queue name, this utility will work as follows:

"""Usage: Marconi Client
...
marconi tail queue_name - lists the last 10 messages in a queue
marconi tail -n N queue_name - lists the last N messages in the queue
marconi tail -f queue_name - lists the last 10 messages in the queue, and then all new messages that arrive
"""

Bonus points for filtering and highlighting.

Takes advantage of some configuration file to avoid having to specify marconi_url, keystone_url, keystone_username, keystone_password, etc.