Update parallax.py

Added support for .png images
Renamed methods

previous normal (color keyed) add_surface() method renamed add_colorkeyed_surface.

new add_surface_png() method renamed add_surface() since the normal way should be a surface with an alpha channel.
Demo images now have an alpha channel
Fixed syntax error initing parallax surface

The arguments of the parallax surface __init__() method changed some time ago. This makes the demo run as expected again.
Merge branch pull request #2.

Modifications from original includes change of method names and updating demo images to use alpha channel