Mobs use animate() to rotate

This removes the need for mob sprites to be pre-turned. Mobs that lie down do so with an animation rather than instantly appearing on their side. You can lie down facing THE LEFT OH SHIT. Note: Simple_animals aren't affected, just carbon mobs.

The "lying down" icon_states for everything you could equip are all now unnecessary. They aren't removed in this pr but doing so later will be easy.

The dmi changes are to make xenos all face upward so when they turn, they're lying down somewhat correctly. They're not terribly good in some situations. I expect WJohnson to have a fix for this eventually after yelling at me.

Comments aren't changed. The overlays_standing list is still around because overlays is not a normal list, and BYOND's normal list functions are worth the extra cost of keeping a spare list of images.

Picture: http://imgur.com/E8mqQNd
Sadly photos taken of lying down mobs show them standing but I'm sure this is easy to fix.

Updated with fixes from Giacom's earlier comments.