Moved ACL to a new package. Created subclasses.

ACLMessage now has multiple subclasses for each type of communicative
act (RE, propose and contract net).
Simplified the interface

The Agent interface is now simpler. I decided to just have
a generic "send(ACLMessage)" and the behaviour must be
implemented by the programmer.
Move ACLMessage and removed the singleton class.

I reverted the decision to use a singleton. The library will
be based on an interface instead.