Make Command extend from NSObject

I was using `Command` in a project of mine (https://github.com/victor/whereami) and wanted a `Command` to be also a delegate for `CLLocationManager`, but the compiler complained that it was not implementing `NSObjectProtocol`, which `CLLocationManagerDelegate` declares as extending.

I found that the simplest way was to make `Command` not be a root class, but an `NSObject` instead, so that commands can implement a variety of standard framework delegates. And hence this pull request, to make SwiftCLI more useful (IMHO, of course)