Many key updates (see extended desc.)

- Set up pointer to show firing direction for player
- Now using com.badlogic.gdx.graphics.g2d.Sprite to reduce calculations
during render method (likely to be modified later for increased
flexibility).
- implemented Projectile.java
- Set up a way to manage safe addition and removing of entities
- Set up a way to manage safe creation and destruction of Box2D bodies
* when a projectile would collide with two bodies at once, contactlist
would contain null fixture in second Contact if body was destroyed
previously.
- Large changes to how Entities are created
* Entities are now initialized with data, then created all at once (to
comply with safe body creation while keeping logic easy to keep track
of)
* Initilization steps are clearer after separation of data
initialization and body creation
- Entities are now removed from Box2D world all at once.
- Implemented basic collision handling architecture (might rework as
needed)