BTRScrollView content & indicator insets basic support

"Hey guys, I'm looking at what Jack was talking about."
This is basic support for `scrollIndicatorInsets` and `contentInsets` for `BTRScrollView` (like in `UIScrollView`). I did it for my private project, but unfortunately there is one issue that I cannot resolve, so maybe you could help with it: if you scroll to the top so you see top inset, and resize window, scrollview resets `contentOffset` to 0 by Y, so inset is not visible anymore. 
You should try, just set `contentInsets` to something with positive `top`, scroll to top and resize window.
Thanks!