Add an "invisible images" use-case

[yoavweiss]

As discussed on http://ircbot.responsiveimages.org/bot/log/respimg/2015-03-05#T117019 we need to write down what's required in order for authors to be able to make their images go away for some breakpoints.

@aarongustafson - up for the task?

[aarongustafson]

Ready for feedback (or merge).

[marcoscaceres]

Added feedback. Thanks @aarongustafson for doing the hard work!