Replace Bukkit API dependency with MIT-licensed alternative

(Not ready to pull, opening this PR for discussion/review to see if it is worth developing further.) Replaces the Bukkit API dependency (GPLv3) with a [clean room MIT-licensed alternative](https://github.com/deathcap/BedrockAPI) written using the API documentation javadoc available at http://jd.bukkit.org/rb/apidocs/ for reference.

Relates to #148 #147 in that this would allow licensing the Glowstone binary distribution more permissively than GPL (assuming there are no other relevant dependencies).

Currently Glowstone with this change *compiles* successfully (e.g., all the needed interfaces are present) but is not yet runnable, due to missing method implementations. However I suspect is feasible to reimplement these missing pieces, if there is interest — though primarily an API, "Bukkit" also includes some important utilities in org.bukkit.*:

- [ ] plugin loader (plugin.yml parsing, class loading)