Added deathmatch mode!

Had to do quite a bit of work to make this happen as follows:

1. Dedicated server needs to select its type during startup.
2. Dedicated server needs to tell client what type of game it is.
3. Client needs to load particular map and game logic only for deathmatch mode.

Deathmatch mode is super basic right now.  Up to four players can join the map as usual.  The difference is now the first player to 10 kills wins the game.

I am doing a bunch of debugging right now and will soon start working on a brand new deathmatch level.  The new level will have all new level design, artwork, and music.  I plan to study doom/quake levels for inspiration.

I also plan to add other cool deathmatch gameplay elements such as other weapons and health/shield pickups.