Add makefile. Update readme. Simplify main.cpp.

Add a makefile. It includes recipes for building an executable and cleaning
unnecessary files.

Update the readme. Document dependencies, and how to build and execute the
application.

Copy the contents of init.cpp into main.cpp, thus simplifying the structure of
this program. Only main.cpp `#include`d init.cpp, and it was unlikely that any
other file would need to include init.cpp, so the existence of init.cpp was
unjustified.