Add warning about make clean

Make clean blows up the bin/ directory. This adds a warning about that.
Bump LICENSE year
Enhance make clean

Make clean now is broken into two parts, build_clean and test_clean.
The build_clean target is responsible for cleaning up all generated
files from the marktab build process (invoked with make). The test_clean
target is similar but for all generated files from the test suite
(invoked with make test).
Update readme with usage information