add solution to Problem 3 in Problem Set 4.

Augmented are the following 2 files:

	src/sat/SATSolver.java
	src/sat/SATSolverTest.java

Besides, a few typo are fixed in the following file:

	src/sat/formula/FormulaTest.java

Thank you!
							Lawrence X. Amlord
							  Oct. 31st, 2014
							   Xi'an, China