Add Merchant API

[gabizou]

I know Glowstone is incomplete, but, it can be useful to at least add the notion of trading with Merchants into Glowkit.

I used to have a related PR for this: Bukkit#1077

As you can see when reading the PR, it went through many iterations and changes, and finally was very close to being accepted before the Bukkit shutdown.

[turt2live]

If @SpaceManiac is okay with it, I can add it to the existing list of PRs that are "useless" without implementation (#3).

[turt2live]

Closing as further discussion is best held at #55