Ensure that integral frame rects are used for the popover's location

This fixes sub-pixel blurring which could occur when displaying a
popover
Ensure the popover displays on top of any other views

For example, if you display a popover from a toolbar button, and the
toolbar is within a popover itself (e.g. from a UISplitViewController's
portrait-oriented side view), it would previously display behind rather
than in front of the existing view.
Merge branch 'master' of git://github.com/werner77/WEPopover

Conflicts:
	Classes/Popover/WEPopoverContainerView.m
	Classes/Popover/WEPopoverController.m
 * In iOS7, WEPopover now appears more similar to the iOS 7 UIPopoverController
 * Use the improved black popover appearance from the demo by default (iOS 6 and below)