Fix detection of path conflicts.

 - os.path.commonprefix does not work as expected to return
   path component prefixes, but rather works on character
   by character basis.
added some more unit tests to test_abspath_overlap
Fix realpath_relation to consider os.sep and os.altsep.
Fix testcase and implement recent fix with new _is_parent_path function.
Merge pull request #24 from NikolausDemmel/fix-common-prefix

Fix detection of path conflicts.