Don't use custom Dir type in signature

As it assumes coercion which Method::Signatures doesn't support at
present:

https://github.com/schwern/method-signatures/blob/726650ecd/lib/Method/Signatures/Modifiers.pm#L134
Drop unused types

The Gitalist specific types are no longer of use and the custom Dir just
went by the wayside.
Added docs to Git::Gitalist

Approaching distro ready!