Update sip_helper to use python header dirs

This fixes https://github.com/ros-visualization/qt_gui_core/pull/46 and https://github.com/ros-visualization/rviz/pull/788; needed at least on OS X, likely some other platforms as well.
Add Python_ADDITIONAL_VERSIONS and ask for specific version of PythonInterp.
Merge pull request #12 from NikolausDemmel/patch-1

Update sip_helper to use python header dirs