Change StatsButtonCell to a segmented control rather than UIButtons

Allows for a touch down inside change and it just plain looks tappable now.  Still need to finalize some of the changes and tweak the fonts used in the control.
Set text attributes to match original button styling

Did not set the background color to grey as I felt this was counterintuitive to iOS 7.
Removed a bunch of commented out code as well that I didn't want to make its way to develop.
Merge branch 'develop' into issue/1399-stats-segmented-control
Added a 10pt padding around the segmented control for visual separation
Merge pull request #1549 from wordpress-mobile/issue/1399-stats-segmented-control

Replaced UIButtons in Stats with UISegmentedControl