Boss implemented

• 1 Boss zombie was added, spawns after around 2 minutes. Shoots at player and spawns other zombies occasionally. Shockwave is released when boss is spawned along with the HP bar showing.
• Explosives now actually kill normal zombies and red ones in the radius instead of just blasting everything away and dealing no damage (still deals no damage to other types of zombies). 
• Upgrade prices reduced to 25, for better experience playing
• Title screen increases types and amount of dead zombies according to Highscore, max dead zombies shown is at a score of 150 to avoid possible lag
• Upgrade button and healing sound effects added
• Healing powerup now rotates
• You can jump while on trees
• Removed a sound bug with the explosives

This is likely the last major update to this game as I've started working on another project. Maybe minor fixes will be still added.