Reduced the else/if chain for Monsters at line 51 to a single if/else
case.

Added isMonster() to EntityMonster.java at line 7

Added the method getEntity() of the following form:

public CraftEntity getEntity(CraftServer server) {
        return new CraftEntityName(server, (EntityName) this);
    }

To the files:
EntityZombie.java
EntityPigZombie.java 
EntityCreeper.java 
EntityEnderman.java 
EntitySilverfish.java 
EntitySkeleton.java 
EntityBlaze.java 
EntityWitch.java 
EntityWither.java 
EntitySpider.java