Added Sprites namespace.

Added the Sprites namespace to contain vario
us sprite data, such as sprite locations on sheets. The namespace was started off with the
locations of a left facing Skier sprite and a right facing Skier Sprite.
Skier class now utilizes Sprites namespace for sprite locations.

Skier also uses additional sprite location. This additional sprite is a left facing skier (to compliment the previous right facing skier), and so the sprite is changed to reflect the player's lateral velocity.