Handle empty root in util.relative

Re #128, an empty root passed to util.relative should return `aPath` as-is. This avoids stripping the leading `/` off of absolute file paths.

Since we don't know whether the empty string is a URL or a file path, we cannot make an assumption either way and should return the path unmodified.

See webpack/core#5 for context around this fix. Specifically, https://github.com/webpack/core/pull/5#issuecomment-50284072 and https://github.com/webpack/core/pull/5#issuecomment-50285215.
Super minor nitpick: Change `rootone` into `root/one`. `rootone.js` is used in the above line just to make sure that `libUtil.relative` doesn’t think that it should strip `/the/root` (since both arguments start with that string), because that would be wrong.