Exposing 'VersionPrinter'

Exposing `VersionPrinter` to match the implementation of `HelpPrinter` for the same purpose.

Additionally, I found solidifying tests difficult when `cli` was imported using the full github path (i.e. `github.com/codegangsta/cli`) and therefore changed the import paths to `"."` instead. If this is an issue, feel free to revert it, or let me know and I will.

Cheers,
J