Added style property

Hey! I've been looking for a pull-to-refresh control which would allow me to build something similar to iOS 7 style `UIRefreshControl`. The effect I wanted to achieve was a pull-to-refresh view attached to the top of scroll view and reacting to pull progress. It appears that `SSPullToRefresh` can be used to support this idea quite effortlessly. Please tell what you think about this :) I'm up for modifications and fixes if required.