added iOS 7 support.

bogardon & timothy1ee,

Thanks for the nice library.
I'm using this to support the application that have to run with iOS 5, that cannot use iOS 6's `UIRefreshControll`.

BTW, in iOS 7, I confirmed the `contentInset` of `UIScrollView` under `UINavigationController` may be not `{0, 0, 0, 0}`. Because in iOS 7, the `UINavigationBar` of `UINavigationController` is transparent, therefore `UIScrollView` has initial `contentInset` for this `UINavigationBar` area. In this case, the default `contentInset` is like `{64, 0, 0, 0}`, and also `contentOffset` is not `{0, 0}`, but like `{0, -64}`.

For supporting iOS 7, it's not always true that we can set `0` to `UIScrollView`'s `contentInset` to restore `UIScrollView`'s state in `finishRefreshingDirection:animated:`.

Can you check this pull request?

Thanks in advance for your supports.
honishi