Some changes

Since the last update, I've been working in internal changes (classes' organization, etc..) but I also achieved important things:

- Basic HUD (a little bit ugly...)
- Screen messages
- Remove double-jumping
- Added provisional footsteps sound
- Changed the balls style. Now they are fire balls and you won't be able to touch them.
- Sprites for starting and ending doors.

I think that's all for now.