Merge: jwrapper: An extern class `in "Java"` generator

Consider as Work In Progress. This is an overview of what has been done so far on the java wrapper generator. Most of the features are implemented, but there's still a lot of work to do. In its actual form, the tool takes javap output stored in a file and wraps all methods and attributes (not really useful, but implemented) to generate a nit extern class in a .nit file. Up to one collection parameter and one collection return type are copied. Methods containing non-convertible types are auto-commented.

Features to come :
* Auto-generation of extern class for non-convertible types
* User interface with options
* Static overload support

Pull-Request: #571
Reviewed-by: Jean Privat <jean@pryen.org>