New path library for dttools.

Moves some old path specific functions from stringtools to the new
namespace. Some cleanup of old functions with tests. Also a new useful
path_absolute function.
Corrects path handling in Chirp.

Chirp now always operates from the root directory.
Fix some bugs with path handling.
Correct errors on Darwin.
Remove old code.

The code changed the url path to an absolute path. This is done in cfs_reinterpret.
Merge pull request #172 from batrick/chirp-path-bugfix

Chirp path bugfix