Adjust child scroll view insets for status bar

Here's a non-hardcoded fix for the overlapping status bar bug on iOS 7 (#153), applicable when the side view(s) are `UIScrollView`s or `UITableView`s. It properly adjusts the inset when the status bar changes for an ongoing telephone call.

I'd prefer to just pass the `topLayoutGuide` down to the children, but it's `readonly`.