Responsive Image Upsizer Improvements

Hey russ, did a little bit of an overhaul on our plugin, which included adding some new options and modifying the ability to use a masking layer in front of the image for cropping. This might be a little too specific direction for this plugin but it might be decent functionality further down the line. The settings admin is sketched out pretty roughly and could use a tiny bit more love as well, but it's some sort of direction.