Bugfix: Chaos wizard didn't have the leech ability
Tweak: Vampires leech more
Tweak: Max chains is now a parameter for lightning bolt
Bugfix/Tweak: Spellcasting monsters did too much damage and had insanely large ranges relative to the player.

This commit also unifies the method of determining whether a monster or the player know a particular spell, and with what paremeters it is known.
The teleport spell no longer requires a parameter; the lack of one means it is player-cast, rather than scroll-cast. (Monsters can't cast it regardless.)
Gasp! New monster!
Tweak: Reduced physical damage rolls at higher levels
New spell (invisibility) and backstab (requires invisibility)
Tweak: Message when a new spell is learned, to make it more clear that it has happened.