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 paths.

This treats root paths of '' and '.' the same.

Root path of '/' will strip the leading '/' from any path.
Merge pull request #129 from jgoz/relative-empty

Handle empty root in util.relative