Add CFLAGS/LDFLAGS/human instructions for building
Rename DelPicosat method to Delete for less stutter
Replace add-one-clause method with add-many-clauses method
Fix off-by-one that had caused crashes
Remove Picosat.SetSeed

I couldn’t figure out how to test it, and the docstring itself says it’s not
useful for industrial problem sets.
Correct typo
Switch from using decision_limit to propagation_limit

Pycosat uses the latter and gives a test for its implementation.
Add basic tests