Alien hunter leap ability

Alien hunters can now leap, Hitting a mob stuns it, Hitting a dense object stuns the alien for a short time.
Uses WJ's glorious Alien leaping sprites.

(Hud code a shit.)

Uses a modified throw_at() that doesn't restrict movement to cardinal dirs, this is so the alien doesn't look fucking retarded when it moves on diagonals.

Aliens in "leap mode" cannot attack, this is intentional.
Aliens in "leap mode" now show a different icon, so they look hunched over/ready to pounce.

http://i.imgur.com/mHyX9wg.gif
Slightly old gif, doesn't show the UI button.