Drops added and various other improvements

- Enemies now drop hearts or ammo with roughly a 2 in 10 chance
- Player can pick up drops and the appropriate action will be applied to
the character
- Major improvement on commenting across all files
- Explicitly setting privacy on functions for readability
- Added placeholder art for drops
- Improved collision system
- Expanded on layer system to assist in object checks and collision
- Lowered chance of a bug which results in the bullet to pass through an
enemy