py: Add Missile and MissileTower
py: Clicking on a Tower allow viewing status for the specific Tower
py: Make towers gain experience and level up
py: Introduce separateTowers to prevent towers from overlapping
py: Make Enemies shoot bullets and Towers can be killed
py: Add HealerTower and healing effects
py: Add buy buttons to insert new towers to the game
py: Show target range of a tower when selected
py: Implement save/load feature with JSON serialization

The serialized stream text should be compatible with JavaScript version.