Fix sourcemaps

Hey,

I tried to fix sourcemaps with the current libsass/node-sass implementation and without having to include all the .scss files in the sourcemap.

This is basically a proof of concept and my first attempt to create something useful with broccoli and node.js in general, so I desperately need feedback if

a) this approach seems any good at all
b) the node functions used were the correct ones
c) the assumptions i've made about the paths make any sense or are way to fragile

Of course the code still needs cleaning up, no debug output with console.log etc. but this should make it easier to see what's going on and for me to debug if it does not work for you.

I'm open for discussion, here or on IRC.

Best regards,
Domme