EnemyCharacter is now dangerous

Part of the battle engine is now working. The example in main.js creates
an EnemyCharacter with a regular attack and a fireball attack. When
defaultAI is run, the enemy will select randomly between these two
attacks and use them on the player. This will take down the player's HP.
The fireball attack takes down the enemycharacter's MP, and when the
MP=0, the enemy no longer uses the fireball.
Any number of actions or enemycharacters can now be created. Next step
is to give the player the ability to fight back. Seems kinda important
for an RPG.