Added component loader classes.

Note that all entity loaders inherit from the EntityLoader class. As far
as I have tested, it looks like inheritance of public members will work.

We might even be able to generalize the entities in the LevelLoader class
through encapsulation... but I think we should stay away from that.

What I'm planning right now is that we can use the EntityLoader to add
things specific to entities and then these traits will be added to all
Enemies, NPCs, and Player loader definitions. More convenience than design
choice.