amberc.js: add finished_callback
amberc.js: add finished_callback

this callback gets called once all compiler steps
are run
adrian-castravete/amber,matthias-springer/amber-athens,adrian-castravete/amber,amber-smalltalk/amber,adrian-castravete/amber,amber-smalltalk/amber,amber-smalltalk/amber,matthias-springer/amber-athens,matthias-springer/amber-athens