Gphoto2

1/ Return string values from gp_camera_wait_for_event.

1/ Include function documentation in Python interface.
2/ Add gp_widget_get_children and gp_widget_get_choices functions that return
Python iterators.

1/ Include interfaces for more versions of libgphoto2.

1/ Added gp_camera_get_single_config(), gp_camera_set_single_config() and
gp_camera_list_config() functions, if used with libgphoto2 v2.5.10+.
2/ Changed version related compilation flags.

1/ API change! The gp_file_open function now allocates and returns a new
CameraFile object.
2/ Compile without -Werror by default. Use PYTHON_GPHOTO2_STRICT to
over-ride.
3/ Improved compatibility with old SWIG versions.

1/ Fix issue 16 (MacOS install problem).