Add a gitpan directory to the Config.

Also directories for logs and repositories.

Have to remove git directories unsafely, they have read only files.
Disallow empty distribution names.

This protects against many subtle errors, including making paths using
the dist name.
Put the repository directories in the gitpan repo dir.

Also
* Use the AC/Acme-Pony directory structure for #42
* Eliminate Gitpan::Dist->directory, don't need it