Merge: FFI with Java

Merge: FFI with Java

Supports:

* Implementing Nit methods in Java
* Associating a Nit extern class to a Java type
* Callback Nit from Java code

To do next:

* Write the manual
* Support callbacks to super and casts (which also implies have static types in Java)
* Support Java code on Android
* Add more complexe examples, such as using an external .jar

Pull-Request: #396
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>