Make preprocessors take traitlets config
Make preprocessors take traitlets config

- We found that preprossor config was not followed - in particular,
  the ExecutePreprocessor.timeout wasn't noticed when it was increased
  to 120.
- A instructor did some digging and suggested that this change would
  make the configuration work.
- I don't know what the root problem could be, and I don't know if
  this is even needed.  Is this patch useful, or is there a "right"
  way of making traitlets work?

jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader