Added method observeNode()

Setting node as observed based on node name.
To set node as observed you have to know number of outcome,
which is being observed.
Fixed event handler.

Check porting to newer version of DisCODe:
http://robotyka.ia.pw.edu.pl/twiki/bin/view/Software/DisCODe/Porting
Added method clearEvidence()

removing all observarions from network.
Added method changeNodeCPT

allowing to set new CPT of the node with given name
in runtime.