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
tapper nitg en dur marche pas chez moi car il va cherche un 'autre' nitg qui sait pas faire du emscripten :/