Use nonpartial for Action.triggered.connect, to quietly absorb trigger arguments

This fixes a compatibility issue between PySide and PyQt4. In the former,
the Action.triggered boolean argument was silently being ignored. In
PyQt4, however, it was triggering errors about functions being
given too many arguments
Redirect d3po.org urls to new location
Merge pull request #280 from ChrisBeaumont/choose_export_session

Handle PyQt4/PySide differences in action.triggered signature