Add improved horse taming system

[Nevakanezah]

Vanilla horse taming is a fairly binary affair, with horses changing from completely inoperable or entirely obedient instantaneously. In addition to being unrealistic, it's not dreadfully interesting.

Implement a system where horses have a spectrum of tamed-ness that applies once they've had their vanilla "tame" flag set. Newly-tamed horses may be harder to control, while spending a great deal of time riding the same horse may yield attribute bonuses and other effects.

This system may also be able to control how frequently horses rear up, a behaviour that can be quite irritating in vanilla, as they cannot move while rearing.