Release 0.6.21

Release notes 0.6.21: -Add a button on the skill tree to allow players to reset their skill points -Disable the overheating mechanism on the HEAVY MACHINE GUN -Fix AUTO SENTRY not targeting reliable through SPIKE BARRICADE -Fix collisions on some trees in the Cemetery map so the player can't walk up itself -Fix free upgrading for the building upgrades -Fix players seing themselves when turning quickly on the HEAVY MACHINE GUN -Fix the XP going back on itself at the end of the game -Increase build phase duration to 2 minutes -Increase the amount of money the player throws out to 250 -Potentially fix the HEAVY MACHINE GUN being used by 2 players at the same time -Reduce the overall recoil of the guns by 30% -Show a message in the chat when a building gets upgraded -TANK TRAPS take only 50% of explosive damage