v2.4.0

1/ API change! gp_file_open no longer creates CameraFile object. It is now
consistent with the C API.
2/ Deprecated functions: gp_abilities_list_append, gp_list_populate,
gp_port_info_list_append, gp_widget_add_choice, gp_widget_set_info,
gp_widget_set_name, gp_widget_set_range, gp_widget_set_readonly.
These will be removed in a future release.
3/ Change licence from GPLv3+ to LGPLv3+.
4/ CameraList behaves like a cross between Python list and dict types.
5/ Added unit tests to check most libgphoto2 functions.
6/ Include virtual camera in "binary wheels".