Fix Firefox by deducting diff if text at load

Testing in Firefox (Mac) I don’t seem to need to reset the diff at all.
Might prove otherwise on Windows but I’ll test shortly. Need to deduct
diff from scroll height if textarea contains text so the correct height
is set, otherwise Firefox has the textarea appear a little larger. Also
use height 0 when resetting based on @fmal’s advice (need to test at
home for IE).
Update demo textarea min/max height values
Add pending 0.4.0 release notes