First of all, thanks -- this is a life savor.
Second, for user documentation, you might want to specify that height and width should not have units. I discovered that the hard way -- luckily breakpoints work.
Although browsers allow them, the spec for img height and width attributes have never allowed px units, they should always be a "non-negative integer" (http://dev.w3.org/html5/markup/img.html).
It would be fairly trivial to support them, but I don't think I want to add a bit extra weight to the plugin to deal with this edge case.
Fix #35 by mentioning unitless attributes. Other minor changes