Add BackPAN::Index::Release->short_path.

authors/id/X/XY/ is a pecularity of the CPAN file layout.  For other
CPAN tools it's often easier to refer to just the CPANID/PATH part.

Also...
* Fix a doc type BackPAN::File -> BackPAN::Index::File.
* Document that Release->dist is an object.
* Test the Release accessors.

For #27