Make path to source files relative

Tested with many combinations in browser. :ok_hand:.

Doesn't normalize Windows paths yet. Putting up for review but want to add that before merge.
Consider using [`uri-path`](https://github.com/UltCombo/uri-path) instead of `slash` in order to properly encode the relative URL paths. See these discussions: https://github.com/gruntjs/grunt-contrib-uglify/issues/222#issuecomment-47471524 and https://github.com/gruntjs/grunt-contrib-uglify/pull/175#issuecomment-37899694