Add support for relative paths in SourceMaps

As of now, I feel getting sourceMaps to work properly is more of a challenge than it should be.
I believe things would be a lot easier if this `grunt-contrib-uglify` could generate relative paths in sourcemaps (and a relative `sourceMappingURL`) out of the box.

I just put something together that "does the job for me". Maybe we can start from here and add proper support for relative paths?

PS: This should solve #39 and #71.