This version fixes a few things after using review.

I still have some open questions:

o Where in unittest.rst should the StopTestRun documentation actually live?
  I've put it right after the SkipTest exception.  That doesn't feel perfect, but I didn't see a place where it would fit any better.

o What should the flag structure be?
  I've implemented it with two flags on the Outcome object which lead to an optional flag on the result object.  It works, but it's a bit messy


o What word should "haltTestRun" really be?    
  I've chosen the word "halt" to represent the "halt-reporting" flag.  I think my firmware background is showing.  Any suggestions for a better term?