Switch to ARC + add better names for the archive iObjects

As pointed out by @lgarron, the iObjects for `compress using...` were named after the UTI. Not very user friendly.
I've switched things to use the extension (how it was), but to also use the UTI description for the details.

Secondly, I've switch to ARC and made things a bit DRYer

P.S. if you're wondering why I switch to `tbz2` from `tbz`; it's what's returned as the 'preferred tag for class' when using `UTTypeCopyPreferredTagWithClass(ident, kUTTagClassFilenameExtension)` (i.e. `tbz2` is the preferred file extension