Add code of the first part in section 3.3.4.

Included is the following file:

	3.3.4_A_Simulator_for_Digital_Circuits.scm

Thank you!
							Lawrence X. Amlord
							  Oct 11th, 2013
							  Xi'an, China
Update the code in section 3.3.4.

Code of the subsection "Primitive function boxes" has been added.

Modified is the following file:

	3.3.4_A_Simulator_for_Digital_Circuits.scm

Thank you!
							Lawrence X. Amlord
							  Oct 11th, 2013
							  Xi'an, China
Update the code in section 3.3.4.

Code of subsection "Representing wires" has been added.

	3.3.4_A_Simulator_for_Digital_Circuits.scm

Thank you!
							Lawrence X. Amlord
							  Oct 11th, 2013
							  Xi'an, China
Update the code in section 3.3.4.

Code of the subsection "The agenda" has been added.

Modified is the following file:

	3.3.4_A_Simulator_for_Digital_Circuits.scm

Thank you!
							Lawrence X. Amlord
							  Oct 12th, 2013
							  Xi'an, China
Update the code in section 3.3.4.

Code of the subsection "A sample simulation" has been added.

Modified is the following file:

	3.3.4_A_Simulator_for_Digital_Circuits.scm

Thank you!
							Lawrence X. Amlord
							  Oct 12th, 2013
							  Xi'an, China