Merge pull request #538 from mcmonkey4eva/master

Pull requests, yay!
Allow only spawned NPCs to chat. (Otherwise stacktrace error)
Add a 'player edits book' event

an event for when a player edits/signs book, see added meta for infos.
Also fix a few typos
Add book script meta+add 'signed' option to books

use 'signed: false' to make the item a book_and_quill instead of a
written_book. Thus reducing need for item script / book script combos.