Add ability to remove pull-to-refresh from scroll view

Sam, I came across the need today to be able to add and remove the pull-to-refresh functionality from a scroll view based on a user preference. The code in this pull request allowed me to easily do that. If you don't have any concerns with this approach, I'd love to see this merged into the master branch. If you do have concerns, I'm happy to update and send a different pull.

Thanks for sharing this great component with us!

john