Added service locator static class.
Added base exception type.
Added stub services.

Interfaces and null implementations for AssetManager, Config, and Logger
service objects.
Moved Exception to util module.
Added SDL2 flags to Makefile.
Added Core::Clock.

main() now also creates and runs this new class, and Core::ExitCode is
now used for exit codes.
Merge feature branch 'core'

This feature introduces the core of the engine.  A base exception class,
a service locator system with some stub services, and a framerate clock.