fix UI bug on iOS 7 iPhone

![ios 2014114 7 10 55](https://f.cloud.github.com/assets/1240920/1917357/452affa4-7d93-11e3-9a6d-5f6d9a2e70ce.png)
![ios 2014114 7 10 53](https://f.cloud.github.com/assets/1240920/1917358/4d22e5a0-7d93-11e3-9952-5bfcc55b13c5.png)

There would be a black rectangle below the toolbar when first loading, and disapeared after anything loaded.
Loading a blank page before real address could be a compromise solution.
If you have any better idea, that will be great.