Make sourcemap source paths relative to file.base; fixes #92.

From [`gulp-sourcemaps`]((https://github.com/floridoo/gulp-sourcemaps#plugin-developers-only-how-to-add-source-map-support-to-plugins)):

> Important: Make sure the paths in the generated source map (file and sources) are relative to file.base (e.g. use file.relative).

However, the generated source map from Libsass contains paths relative to the file itself, not `file.base`. So to mediate between the two libraries I think `gulp-sass` has to change the paths that come form Libsass, make them relative to `file.base`, so they are correct for `gulp-sourcemaps`.

All the existing tests still pass, and I added a new one for this particular situation.

This fix seems to take care of the problem I described in #92.