Make `show{view,layer,image,imageref}` work on device

Right now the `show{view, layer, etc}` commands save the image to a tmp directory using `NSFileManager` and then open it locally. This works fine for the simulator where Cocoa and python share a filesystem, but on a device we can’t get at the saved file.

I’ve got a little proof of concept that grabs the image data off of the device and saves it locally via python. I’ll clean it up and submit a pull request, but wanted to start a tracking issue and get any feedback sooner rather than later.

(Also, this is basically just a change to `_showImage` so should be compatible with #17.)