Use the faster C++ version if available, unless the caller specifically requested
the Python implementation.

This is considerably faster on my machine.

Using SConstruct hasn't panned out. SConstruct introduces too much of
its own boilerplate code and regular makefiles are understood by more
people. Also, SConstruct needs to be installed separately, whereas GNU
Make is available virtually anywhere.

Based on the code available at http://www.cs.unc.edu/~bbb/diss,
this is a cleaned-up version of schedcat intended to serve as
the basis for all further development.

Notable improvements: more unit tests and a cleaner module structure.