Added support for flipping Actor images horizontally and vertically.

[AnthonyBriggs]

This is a (hopefully) straightforward change which exposes Pygame's transform.flip in an easy to use way.

• set Actor.flip to True/False to flip horizontally
• set Actor.flip to a (horizontal, vertical) tuple to set both axes

This seemed cleaner to use than having separate properties for each axis, eg. actor.flip_horizontal and actor.flip_vertical