Entities and color blending modes

Added an Entity system. This system is designed to be a hybrid of a
component system and a hierarchical system. Components can be added to
Entities, but Entities are mostly designed to use an inheritance model.
This may change in the future, but it makes writing code very easy for
now.

Added a new element to the rendering system to allow a user to choose a
color blending mode or add custom modes.

Changed Hashtables to HashMaps
Camera Support

Added the ability to add an Entity to the Game to be treated as a
camera, rendering everything at its position. Also created the
FollowerCamera class, which follows another Entity around using a
strength variable that decides how strong its connection to the Entity
will be.

In Entity, renamed pos to position and made it final. Now anything can
set a Vector to be equal to an Entity's position and they will always be
referencing that position.

Added a getImage() method in Screen, because screw it, I want objects to
render themselves however the hell they want. I'll probably include
methods to switch between world space and screen space later.