Create packages for tagged stable package versions

This should resolve #7.

This branch merges pull requests #851 and #1086 with the master branch. I've also fixed a bug where stable packages were being written to the wrong directory (they were written to packages/ instead of packages-stable/).

With this code, it's now possible to do:

    $ make STABLE=t recipes/magit

and the file packages-stable/magit-1.2.0.tar is created.

This is the only thing that stops MELPA being perfect for all my use cases, so I'd love to see this merged. Let me know if there's anything I can do help push this feature out the door.