Add Rectangle class
Fix build script. Still needs much improvement
Use our own Rectangles instead of SDL_Rect.

Their purpose is mainly to facilitate creation. Also, with them we
don't need to keep passing pointers around.
Use conversion operator instead of explicit GetHandle function
Rename log.h to Log.h
Add initial jewel implementation

These files are probably on the wrong place. Maybe they should
be moved to a 'src/game' folder?