Ensure that action sheets are included in accessibility paths. (refs #46)

`UIActionSheet` is a class that forces its instances' presence in the accessibility hierarchy
even though they are not themselves accessible elements. Without including it in the
hierarchy, action sheet buttons cannot be manipulated by Subliminal.
Identify popover views by their background views. (fixes #46)

Popover views' superviews do not carry the accessibility label "dismiss popup"
when those popovers show action sheets. Popover views always have
background views that inherit from `UIPopoverBackgroundView`, on the other hand.
Merge pull request #47 from inkling/jeff/identify_actionsheets

Ensure that action sheets are included in accessibility paths. (refs #46...