Add support for blurred background view

Added support for blurred background view on iOS7 and tinted background
for iOS6. If you don't set it, the view behaves the same as before.

Added support for iOS7 status bar styles by taking into account whether
a background view is displayed or the brightness of the background
color.

Fixed a lot of bugs with iOS6.

Here is how it looks now:
![ios simulator screen shot apr 12 2014 3 29 32](https://cloud.githubusercontent.com/assets/2270433/2685909/a37ee76a-c1da-11e3-8f4f-af6ccb0e7af8.png)