@ashfurrow => fix help button antialiasing

I made some changes in the button pod that are half what you were doing with the help button and half what i'd done before with ARMenuButton to avoid the weird 50% opacity grey around the edge of the button. The changes are mostly in ARMenuButton and can be found here in the most recent 2 commits: https://github.com/artsy/Artsy-UIButtons/commits/master

The trick is to make sure that the black circle ends at the point where the white border begins. That is to say, it's radius is slightly smaller than the that of the border. I'd done this before by drawing ellipses in drawRect. Now I'm accomplishing the same thing with just layers, background, and border. Let me know if there's a better way to accomplish this, I just did what seemed most obvious to me.