Refactor do_update_line in inserting mode to reduce complexity.

Extract get_obj_at_point and to_scene_point functionality into
separate class functions making them independently testable as
well as allowing them to be stubbed in tests.

Extracted time limit functionality for callback functions into
a time_limited decorator that gets used by get_obj_at_point.

Work towards #19
Further refactor do_update_line.

Move get_hightower_object_at_point into separate
GetHightowerObjectAtPoint function object.

Moved grid coordinate conversions into scene.

Work towards #19
AppVeyor badge should only care about master branch builds.