uml: Added UML class diagram generation from a Nit model

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
nit: Added new binary nituml, for generation of UML diagrams from Nit source code.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
nituml: Can generate UML package diagrams from nit source.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>