Don't require user to call `removeFromSuperview`

[soffes]

Currently, you have to call removeFromSuperview to prevent a KVO leak. This happens because the weak reference to the scroll view gets nil'd out before SSPullToRefresh can remove it's observer.

Here's my StackOverflow question on the issue: http://stackoverflow.com/questions/10793206/how-do-you-remove-kvo-from-a-weak-property

Any help would be greatly appreciated!