For extensibility, refactored solver.cc into three files --- solver.h, global.h and sat_test.cc.

solver.h is the core algorithm part. global.h defines global typedefs. sat_test.cc defines the former main function.

As I have no knowledge about Rakefile, didn't modify it at all.
Also, I kept solver.cc still for comparing and other goods. However this is duplication to be resolved.