Add "kTopOffset" parameter to make RefreshControl work on Apps with "translucent" header bar

I've implemented the ODRefreshControl into an app that uses the "translucent = YES" setting and therefore the content appears unter the header-bar so all views of the navigation-controller start at the very top of the phone and not unter the header bar.

This parameter can be set to the pixel-height of the header (as a custom-solution might have different heights" or to 0 if the header is not translucent.