Include missing CB libraries

Reorganize dependencies to match https://github.com/Bukkit/CraftBukkit/blob/master/pom.xml
Update README for API
Fix broken null contract with Jukebox.setPlaying, Fixes BUKKIT-3429

The javadocs state that a null may be used to remove the currently
playing sound, however this causes a NullPointerException.

It also doesn't process registering the record correctly, along with
processing non-valid items.