lib/standard/string: Moved escape_to_dot from nitcc to standard/string.nit

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
model: Fix comment for MVisibility

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
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>
nituml: Added tests for new tool

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