Updated status bar height to be dynamic instead of static

Hi, thanks for this awesome library! I ran into the attached issue where I was presenting a banner on a UIWindow but the current view controller had no status bar (via prefersStatusBarHidden). This code seemed to fix it. Feel free to incorporate it or use something better. Thanks!

![ios simulator screen shot nov 20 2013 12 08 36 pm](https://f.cloud.github.com/assets/2292792/1584069/7df69518-5206-11e3-8714-8207fe461b40.png)