Issue/2213 comment moderation punchlist

Improves comment moderation buttons, based on design feedback from @drw158.

 * Buttons are now evenly spaced, and match up with the same horizontal positioning as the tab icons. I used Auto Layout to accomplish the centering and positioning.
 * Unapprove/Approve icons updated to match o2/wp.com notifications
 * Button label color changed to match button color.
 * Added some padding to the bottom of the moderation views, so the buttons don't bump up against the app tabs when there's a long comment.

![screen shot 2014-08-11 at 1 51 08 pm](https://cloud.githubusercontent.com/assets/789137/3882667/4cb25772-2199-11e4-93d0-55647c776fd3.png)