nitdoc : changed redef methods in MModule

added suffix "get_" before each redef methods in MModule and make them return a formated String
fixed call to this methods in nitdoc

This will improve the futur use of doc_layout for the main content of the html page

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>
doc_layout : introduce classes for the main content of the page

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>
nitdoc : use doc_layout for html generation on an overview page

graph : changed process_generate_dot method in NitdocOverview and
generate_dot in NitDocPage so they return a String and can be used
in doc_layout. This make graph not displaying on every other page
except the overview one, but next commits will fix this.

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>