(1) Added the OpenGL ES 2.0 header files to the libraries folder.

(2) Restructured the libraries folder to lowercase and to just holder header files.

(3) Switched system creation order, so WindowSystem is first, then GraphicsSystem.

(4) Adjusted tabs to 2 in some files.

(5) Added forward declarations for WindowSystem and GraphicsSystem in Engine class for future accessors. As opposed to using global pointers, which some ES can't support. And its a bad coding practice.

(6) Added <functional> include for using std::functions.

(7) WindowSystem now setups OpenGL functionality. This will need to be in a #ifdef when using DirectX?

(8) Xcode project now compiles using C++11 compiler, also looks inside library/ folder for lib header files.