Creating an MRScalingImageCache podspec

Since I was tidying up my own libraries, I thought I'd turn this one into a pod too. There are no source code changes to enable this, simply a shuffling of files and directories to make it a little cleaner once you start adding in license and podspec files.

You'll want to tweak the 's.source' git repository if you decide to take the change to ensure it's pointing back to your repository.