There is now an identifier to define which type an enemy is. Tier one is normal, tier two is laying mines (nyi) and tier three is one that shoots bullets which follow their target. Out of three of the enemies there is now one tier three and two tier one enemies. It is now possible to set bullet speed in the constructor. You can now specify the enemy type (which tier) in enemy constructor. It is now possible to assign different .bmp files (skins) to enemies. Got rid of some warnings and useless code. Updated TODO.txt.

Little explanation on the tier three tank:
- Their bullet is only fired if the pipe of the enemy is at that very moment pointing straight at the player it's trying to shoot at.
- Their bullets go at a speed of 2 rather than the usual 5 because else the players can't outrun them (they run at speed 3).
- Their bullets have only one life.
- These tanks have a different skin from others (duh).
- I probably forgot to mention some stuff but it's been a long day... :)