Refactored BrokerAgentBehaviour for experiment 03.

The method 'getProducerPayload' adds itself the payload of the current
interval to the result. Previously this addition was carried out out of
the method.
Simplified the trackering of Distributed Events.

Now the method only tracks the values of the intervals that are applied
as part of the event. Before this change, the tracker also logged the
date and duration of each interval.
Added support for multiple Directory Facilitators.

Now, according to the configuration file, many Directory Facilitators
may be instantiated at the same time. Each ASPEM distributes its broker
agents among the DFs. The result is that independent auctions are hold
for each DF. The ASPEM is responsible for managing all of them.

This commit also includes a basic integration tests for this new
feature. It tests the 'auction01' with two DFs.