Added Icons for DarkRP SWEPS

I also fixed the ammo icon (some of you would not have this issue but I
for one did not have the icon for ammo on my client, so I presented a
solution. I don't like where I put resource.AddFile() for it, but I
imagine if FPtje hates it, it will get moved.

I also updated admin only icons. They used to use silkicons, a
pre-gmod13 path. As far as I am aware, silkicons got ported into Garry's
Mod 13 as a bunch of PNG files in materials/icon16. So I just fixed up
the path, thats all.

Another thing I want to do, and I cant think of a generic icon to use,
but I want any swep with a missing icon to use a generic "Hey, I'm a
weapon!" icon.

I may make one later...