Add Citizens NPC Shopkeepers

This PR adds Citizens NPC support. We've already talked about this quite a bit, and to be totally honest I thought we were discussing it in an existing PR- not in bukkitdev chat!

I lost my BukkitDev access a few weeks ago in a botched rename, and I'm not sure when they're going to get it fixed up (frustrating).

In any case- if you're ready for this change, I think it's ready for you. I've been running it on my public server for a while, using a variety of NPC shopkeepers (including Player NPCs), and it all seems to work great- you've seen it for yourself, I think?

Let me know if you want any more changes, and I'm of course willing to help improve the functionality over time- but I think it'd be great to get this in for a near-future update.