Compilation to JavaScript using the Emscripten SDK

Published for review but still needs some doc and examples. Works with most small examples but not yet with the naive_interpreter (but it's not clear why).

Usage:

    apt-get emscripten
    nitg -m emscripten examples/hello_world.nit
    nodejs hello_wold.js
Hum, est-ce `escape_to_c` fait pas l'affaire? en mieux genre en traitant " ' et \