Move OS_interface files, regenerated using cppclassgen.
Corrected case error in directory structor and added files.
Corrected typo in cppclassgen template (and all files it affected).
Added coding_conventions file to help facilitate conistency.
Splitting functionality into multiple classes with single responsibilities.

- Added OS_interface for init/quiting SDL.
- Added window for creating/manipulating/destroying SDL windows.
- Added graphics_context to create/manipulate SDL OpenGL context.
- Added common directory including debug functionality.
- Added fatal_construction_exception to allow immediate exiting of program in an organized
  manner (vs just exit(0)).
- Added engine, which houses the main update loop.