Version 0.7. HUD

- Added HUD class. It's a basic hud that contains the health bar of the
player, the weaponry available and score and stage not implemented. The
HP bar updates every frame on changes in player's heath.
- Again, problems with performance. No idea why.

KNOWN BUGS:
- The source of the bug in which an enemy becomes invincible has been
"found". It happens only when you create two enemies quickly with the
mouse, so don't worry about it for the game as this feature ain't going
to be available.