Sticky iOS7 status bar feature

[itsthejb]

Hi,

Very nice component. However, this suffers from the usual clash between the iOS7 status bar and the side controller paradigm - ie. the status bar untidily overlays main/left/right controllers. As inspired by Mailbox's implementation, I used the UIScreen snapshot trick to stick the status bar to the main view controller: https://github.com/Memrise/LRScrollingSidebarController/tree/feature/ios7-status-bar-snapshot

I think this is much tidier than #6, although it is a tiny bit hacky in comparison.

Note that I didn't do a PR yet because I wanted to get your input first. There are a few caveats; for example this makes the overlay view feature redundant since we need to snapshot the entire main view controller's view - cropping _UIReplicantView using an approach like -[CALayer renderInContext:] seems to be prohibited by CoreAnimation, and just capturing the status bar itself may also not be satisfactory. Also, this should be an option flag and not always on. Furthermore, needs better backwards compatibility.

Anyway, please let me know what you think. 👍