Set up the project directory
Added Rect, Matrix, Vectorf, Engine

Ported the math classes and structs to the engine. Engine now contains
SDL and OpenGL initialization code (tested for Windows and Linux).
SystemManager, Input, Math

(Forgot to add Rect, Vectorf, and Math files in the last commit. Oops!)

SystemManager is a class that contains (or will contain) the "global"
InputEngine, RenderEngine, EngineSettings, and other similar structures.
Derive from this base class for classes that need access to these
systems.

Input has been added in the InputEngine class.