Deathmatch mode, local multiplayer and visual upgrade!

This update adds completely new game mode: Deathmatch, where your goal is to kill other players and don't get eaten by zombies, as well as several other features such as:
- Local mulitplayer
- A new barricade type "Propane Tank Barricade"
- Reworked barricades, turrets and background visuals
- Ping indicator

Hope you're going to enjoy the update, especially the Deathmatch mode!