Update my answer to Exercise 3.31.

Newly added is the following file:

	Exercise_3.31.scm

Thank you!
							Lawrence X. Amlord
							  Oct 18th, 2013
							  Xi'an, China
Update the code in section 3.3.4.

Modified is the following file:

	3.3.4_A_Simulator_for_Digital_Circuits.scm

Thank you!
							Lawrence X. Amlord
							  Oct 18th, 2013
							  Xi'an, China