Set background color of the pull to refresh view.

This pull request sets the background color of the SSPullToRefreshView to the same color of the content view. I think this is the expected behaviour. For example when your scroll view has a white background and you set the background color of your custom content view to grey and pull down the scroll view the color above the content view is still white:

![white](https://f.cloud.github.com/assets/540025/429674/4755e3ae-ae4d-11e2-9f2d-1260d868d4d6.png)
![correct](https://f.cloud.github.com/assets/540025/429673/45243e1e-ae4d-11e2-8bdd-36460753eae2.png)