Rename UIScrollView.parallaxHeader property because of naming conflicts with MXParallaxHeader pod

[yklishevich]

As you can see on the following screenshot http://take.ms/RsgTR MXParallaxHeader pod is also using parallaxHeader name for property on UIScrollView. I experience troubles using the both pods in my project. Could you rename property being added library specific prefix, for example "vg" as I have done in this pull request.