Adds compatibility for gulp-sourcemaps

This PR brings in the new source mapping system that's being spec'd in gulpjs/gulp#356 and [floridoo/gulp-sourcemaps](https://github.com/floridoo/gulp-sourcemaps).

Note: until floridoo/gulp-sourcemaps#4 is merged, the `sourceMappingURL` in the output CSS files will be incorrectly formatted.