Added PlayerBookEvent, PlayerBookEditEvent & PlayerBookSignEvent.

Adds three new events: PlayerBookEvent (abstract), PlayerBookEditEvent and PlayerBookSign Event.  These are fired when a player edits or signs a book & quill item.

Implements: https://bukkit.atlassian.net/browse/BUKKIT-1995
CraftBukkit PR: (pending)
Test Plugin: https://github.com/desht/testbookevents