Add package dependencies

I plan to add this package to MELPA and MELPA Stable, so these dependencies are required. Note that the emacs "24.4" dependency is due to your use of `cl-case`: if you can replace that with something more traditional (and remove `lexical-binding: t`), then the code would probably work fine with Emacs 23 and older 24.x releases, which I think would be a good idea.