Additions to support UIButtons

Hi, I discovered SDWebImage and your categories today to replace AFNetworking basic loading mechanism and its awesome! I needed support for UIButtons and made a fork and private pod to support it. It may be of interest to others!
I chose to share the add/remove activity indicators implementation with a category on UIView, which makes the add method implicitly public if you import its header... I thought it would be a minor side effect in order to avoid duplicating code. Let me know what you think!
PS: This haven't been tested much, I used what was needed in my specific use case.