support retina and non-retina screenshots

Give screenshots on retina displays an @2x in their filenames right after the device name so that the screenshots won't conflict when making a batch run with retina and non-retina devices.

Although iTunes screenshots are all retina now, I also use KSScreenshotManager to generate screenshots with "clean backgrounds" that can be used as the basis for an app's launch images, and launch images must still be done at both 1x and 2x.