Fixes the back icons for the scythe and the Roman shield.

Resolves #2213 

I was doing this for Urist McStation, and figured I'd just push it here instead.

-Fixes the back icons for the scythe and the Roman shield.
-Adds a lying down sprite for the scythe and Roman shield

Now, this does not fix the ion rifle, which was mentioned in the comments of Issue 2213. The problem is that the ion rifle (for some crazy reason) has no in hand sprite, yet has a back sprite. So if someone creates an in_hand for the ion rifle that isn't the standard "gun" I'd be glad to get the back sprites working. HOWEVER, it does fix the issue of the ion rifle turning into a riot shield when put on your back. There's just nothing there.