Added communication skelet.

Data model redefined to better suit communication with Aleph.
Fixed little bug in aleph.py.

Bug caused .getDocumentIDs() to raise exceptions, now it returns empty
list.
Fixed circular import problem in convertors.py.

See http://stackoverflow.com/questions/744373/circular-or-cyclic-imports-in-python for details.
Added AMQPMessage definition for test purposes.

Definition came from https://github.com/jstavel/edeposit.amqp