Fix Enemy Registration with Rarity Tables

[Hamunii]

Fixes #64 by doing the following:

• Adds the Enemy to spawnableEnemies
• Uses levelRarities list instead of spawnLevels for finding out if the enemy should be added to a level
  • This is because spawnLevels is never set when registering using Rarity Tables

[zealsprince]

Looks good to me - applies the fix I proposed. Just tested and appears to work without any side effects.