Develop

Next time I'll push the feature branch to github then do a pull request to develop, then to master.

Recently added: `EntityListeners`

Now, a listener can be set for when an entity collides with another fixture.
Just call `entity.addListener(EntityListener<EntityType>)` with an appropriate implementation of `EntityListener`.

`Player` has one for testing, right now all it does is print when the player touches something.

`EntityListeners` are linked to the entity, so one doesn't need to be set for each body an entity may have. However, each of that entity's bodies will need to have its userdata set to that entity.