Fixed a bug related to mobile images in both {img} and {mobile-img}, added config option to disable placeholders

 Both {img} and {mobile-img} still had references to the old ctx.responsive_styles framework which was replaced recently by the MediaQuery object.  Updated both classes to use the new Element.add_rule method.

 Also, updated the image test and added a MobileImage test to verify that mobile image functionality.  Added a configuration setting allowing Inkcite's default placeholder functionality to be disabled (it is enabled by default) and now the tests specifically turn it on to test that functionality.