copy background attributes from existing clip view to JNWClipView

I noticed that when I tried to set translucent backgrounds on my JNWCollectionView that they seemed to be backed by a white layer. I tried various methods to make sure the scroll view wasn't opaque, but my debugging led me all the way down to the submodule of my project's submodule, so I figured I'd pull request from there.

Context: I'm using Xcode 6.1.1 and building for OS X 10.10
Also can we change this init style over to the pattern used in `-initWithFrame:`?