Made a few changes to the structure of the existing code and added my bits in.

Added an extra state to Game1.cs for the clay pigeon level and added key
bindings for F1, F2 and Escape. F1 switches the game to target practice,
F2 switches the game to skeet shoot, and Escape exits the game. This is
just an initial commit for my part. I'll push out some more updates over
the next few days with fixes, collision detection for the pigeons, and
other code changes. -Mike