cab get and cab outdated --future

This solves two issues:

* Adds the `cab get` command, corresponding to `cabal get`. `cab unpack` is an alias. Solves #24.
* Configures `cab outdated` so that it properly analyzes the relationship between an outdated package's version and the Hackage version (i.e., it uses Ord). Enabling the `-u`/`--future` switch will also display packages whose versions are greater than those on Hackage. Addresses #17.