New towers

Added two new towers:

• Sniper: A long range tower with high damage and low firerate.
  
• Freez: The first area of effect tower. Slows enemies down in its range. Does not stack with other freez towers.

Other changes and fixes

• The player now places towers on a grid. The grid can be toggled in the bottom-right corner
• Added particles to the game
• Fixed a bug where pressing the same wave multiple times before disapearing  would give money on every click
• The player can now see the cooldown of a tower
• Enemies now destroy towers in their way