trappy: allow running the testsuite in environments without ipython
nose tries to import all init.py files. Even if you do in trappy/init.py:
try: import trappy.nbexport except ImportError: pass
The testsuite fails because it then goes on and tries to import trappy/nbexport/init.py. To avoid that, let trappy/nbexport/init.py fail gracefully if it fails to import anything needing for the export.