Gphoto2

1/ Allow user data to be passed to gp_log_add_func (and then to user callback
function).
2/ Add some member methods to GPPortInfo.

1/ Fix a few minor bugs.

1/ Fix bug in gp_camera_get_storageinfo when camera has more than one storage
card.

1/ Further changes to Python interface to logging callback functions.

1/ Reworked Python interface to logging callback functions.
2/ Renamed submodules and revised build process.
3/ Hid xxx_ref, xxx_unref & xxx_free functions from Python.
4/ Fixed some bugs in error handling.

1/ Temporarily fix build problems with Python 3.5
2/ Added a "choose camera" example showing how to handle multiple cameras.