Add support for "$forceEvaluate" to bypass the gate for a single digest ...

- As it is written, there is no way for a data binding inside a directive
  to trigger an evaluateOnlyWhen that is outside of that directive,
  because the two-way data binding won't ever get called

I'm not at all convinced that this is the best way to do this, but it's the way I found that would work.  What do you think? Any thoughts on a better way to solve the problem?

Basically I want to only update a section of the page when a given value updates, but unfortunately it's sometimes updated from within a directive which would normally use 2-way data binding.