Added Makefile dependency files to .gitignore
Added Clock service interface and null object.
Added service locator.
Added methods to Service::IClock.
Added SDL flags and libs to Makefile.
Added Services::SDLClock
main() now provides and runs an SDLClock.
Filled out SDLClock.run()
Merge feature branch '_clock'.

This branch implements a Clock service that provides framerate control
and executes the game loop.