cab get and cab outdated --future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* Configures `cab outdated` so that it properly analyzes the relationship between an outdated package's version and the Hackage version (i.e., it uses Ord).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.

Edit: I also replaced the `system` function from the `System.Cmd` module, which is now deprecated.

Edit 2: I noticed an issue with `cab outdated` and `cab search` that would cause them to hang on Windows. It turns out this can be fixed by enabling `-threaded`, so I changed the `.cabal` file to use it only if Windows is used.