Introduce differents generation mode

[mcg-web]

Now the generateClasses and generateClass accepts bitmask to do more complex work when generating classes:

• MODE_WRITE create dirs and files only if not exists
• MODE_OVERRIDE allow to override file if existing
• MODE_MAPPING_ONLY returns the class mapping but nothing is generated
• MODE_DRY_RUN play all the generate process but no dir or file are actually created
• true is shortcut to MODE_WRITE|MODE_OVERRIDE
• false is shortcut to MODE_WRITE (default)