modify make_screenshots.py to invoke WaxSim correctly for iPad

WaxSim needs to have a "-f ipad" command-line switch when running for iPad…it is not sufficient to modify the NSUserDefaults for com.apple.iphonesimulator.

I'm not much of a Python programmer, so there is probably a more elegant way to do what I'm doing.

Note that the sample project is phone-only, so the screenshots will not work correctly if add iPad to the device list.  However, you will be able to see that the simulator is correctly launched as iPad.

I have verified on universal apps that the screenshots do work correctly on iPad with this script change.
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.
Merge pull request #2 from chucks/ipad_support

modify make_screenshots.py to invoke WaxSim correctly for iPad
Merge pull request #3 from chucks/retina_filenames

support retina and non-retina screenshots
Made iPad lowercase to match iPhone prefix.