Fix #221

IE seems unforgiving of double focuses
Fix #242

Firefox 32 appears to get rid of empty divs so if we have
<div>a</div><div>b</div> and we delete the b the entire 2nd div is
gone. It's impossible for us to know if the user backspaced twice (which
would get rid of the 2nd div in other browsers) or once. Thus we now
completely take over backspace and delete behavior.
Fix #246 and add test
toughen comment strip test
prevent setHTML(' ')
use trim()