webish.py: handle asynchronous checker results.

webish.py: handle asynchronous checker results.
Thanks to robk for pointing out that Nevow will accept a Deferred almost
everywhere. In this case, we just pass a Deferred into ctx.fillSlots(). One
quirk: nevow doesn't evaluate all rows of the table in parallel: using a slow
Deferred in a slot in one row seems to stall the next row until that one has
fired, probably to simplify the flattening of the HTML.