Tower Survival: Hammerstorm

- Added a guide that explains how the game works
- Removed duration and knockback. They only made a difference at higher levels and their effect was not very clear. They were also difficult to balance.
- Introduced 2 new stats:
- Critical hit: increases the damage dealt when hitting enemies with a hammer's head.
- Repair: makes the tower recover hp after the end of a stage.
- Made critical hit numbers be more clear on the screen.