Add a popover background view and sizing options for showing on iOS 6

- Added DDPopoverBackgroundView directly to the project to avoid any additional dependencies
- Updated DDPopoverBackgroundView to implement wantsDefaultContentAppearance to avoid drawing the inner shadow of the popover
- updated OSKPresentationManager to set both the popover background class and the popover size from the sheet controller's preferredContentSize
Adjust coloring of OSKNavigationController bars

- Only set tint color of navigation bar on iOS 7+ where the tint color does not change the entire bar color
Selectively set the text field alignment to support known issue in iOS 6

- iOS 6 does not support NSTextAlignmentNatural on UITextField, so use NSTextAlignmentLeft there instead
Text field fixes

- Fixed the content alignment of the text fields so they center in their cell
- Only set tint color on iOS 7+
Remove iOS 6 grouped table view background color

- Make the default background view for grouped table views invisible on iOS 6 so the right color can come through
Fix crashing bugs on Pocket view controller

- Selectively call UI methods in the Pocket view controller that are unavailable on iOS 6
Fix attributes not compatible with iOS 6

- Check to make sure setting attributes on view controller only happen on iOS 7+