Convince bdist_wheel to package the nevow_widget dropin file.

Fixes #35

I should have done this after #34, no doubt.  Still, I manually verified that sdist and bdist_wheel both produce packages containing nevow_widget.py with the change in this branch.  I also actually installed both packages and ran the test suite with both of them.  The results are the same except for some skips due to *other* files that get left out.  But they're examples that I'm not as concerned about.