Reverted Elephant stuff.
For now, Elephants can't check through units. Gotta be vigilant about
that one.
Fully functioning Empowered movement.
Empowered units don't threaten or check. Will fix that next.
Bishops and rooks empowered by knights now check.
Fixed the sprite filenames. Didn't commit it earlier like I should have.
Empowered knights and bishops now check.
All Empowered pieces check from all combinations.
Reverting changes made in master. Will redo later.