Fix status bar overlap on iOS 7

This uses an adapted version of this code [1] found on SO. It has been
modified so that the existing code (which performs a similar status bar
fix for iOS 6) continues to operate for those using an older version of
iOS.

I originally attempted to fix this in the javascript, but the way
junior.css uses position: fixed to layout the page seemed to resist my
attempts to change it.

[1] http://stackoverflow.com/a/19249775/372120