Added podspec for MBPopoverBackgroundView

Custom `UIPopoverController` background view for iOS.
Implements custom subclass of `UIPopoverBackgroundView` as described in iOS Developer documentation

* [UIPopoverController Class Reference](https://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIPopoverController_class/Reference/Reference.html#//apple_ref/occ/cl/UIPopoverController) 
* [UIPopoverBackgroundView Class Reference](https://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIPopoverBackgroundView_class/Reference/Reference.html#//apple_ref/occ/cl/UIPopoverBackgroundView)
* [View Controller Catalog for iOS - Popover](https://developer.apple.com/library/ios/#documentation/WindowsViews/Conceptual/ViewControllerCatalog/Chapters/Popovers.html)
* [iPad-Specific Controllers](https://developer.apple.com/library/ios/#documentation/WindowsViews/Conceptual/ViewControllerPGforiOSLegacy/iPadControllers/iPadControllers.html)