Added sign sprites and condition for the menu to be found.

- The menu will be accessible only after it is found in game through
  the first puzzle. Not being able to save is negligible during this
  period since the player hasn't accomplished anything yet.
- Sign sprites still need to be made collidable and interactive to
  display their message. These can be used for puzzle hints.
Added collision detection function, but no actual collision.

- See collide() for descriptive comment of object requirements for
  use. TL;DR: Objects need x, y, width, and height properties and
  should be anchored in their centers.
- Gave lucas width and height properties.
  * The lack of width and height on lucas went initially unnoticed and
    resulted in some perplexing collision output. "Why is
    this.x+this.width/2 not a number?!"
- Set anchor of signs to their centers.
- Might be helpful to have a generic constructor for sprites.
Added sign_post (a semi-original) and some other stuff.