Added methods to IDirectory
Corrected ForgotMessage and UnknownPeer exceptions

- ForgotMessage: Removed the target argument
- UnknownPeer now inherits from HeraldException
Added methods to IDirectory, and avoid arrays

- Replaced Peer[] results by Collection<Peer>
- Added method to get the peers belonging to a group or a node.
Corrected signatures of IHerald and ITransport methods
Added a constructor to the Target exception

- Target(Peer) => calls Target(String)
First implementation of Herald core in Java
Small corrections in Herald Python doc & code

- Corrections made when porting code from Python to Java