Add enemies and AI hooks

[mdkess]

Naturally, any good game needs a solid dose of murder. The goal of this ticket is to facilitate it.

My idea for managing this was to have three separate parts:

1. GameEntities. Eventually these will be moved to be called PhysicalEntities. These represent an object's physical state - mass, velocity, forces, etc.
2. InputHandlers. These are given a PhysicalEntity, and their job is to manipulate the physical entity's state (by applying forces, for example) based on their internal logic. A user input handler would translate buttons/touch events into forces. An AI input handler would use its internal logic to update the physical state. Which leads us to...
3. Sensors. This is how InputHandlers see the world. Sensors can be anything from keyboard/mouse/touch sensors (in the case of a human input handler) to rays or pressure points that the AI can see the world through. Thus, the AI can use its sensors to query the state of the world.