[#66] added a default melonjs jsdoc3 template and modified the corresponding gruntfile

note :
- doc generation is still broken (but as i'm working offline without the
online jsdoc3 documentation, i'm stuck !)
- the "new" jsdoc3" melonjs template is the same as the default template
(waiting for @DbLK?)
- old melonjs jsdoc2 template has been moved to the same directory