Added two callbacks and a new method.

Callbacks:
* LibSpellbook_Spell_Added(id, bookType, name)
* LibSpellbook_Spell_Removed(id, bookType, name)

Method:
* lib:HasSpells() returns true if the spellbooks have been scanned successfully.
Typo in LibSpellbook:IsKnown.
Do not scan spell tabs of inactive specs.