fix bug in arc.py of Problem 5 in section 4.3.

The following file is modified:

	arc.py

Thank you!
							Lawrence X. Amlord
							  Apr 12th, 2014
							  Xi'an, China
add solution to Exercise 4.2.

Included is the following file:

	flowers.py
	polygon.py

Thank you!
							Lawrence X. Amlord
							  Apr 13th, 2014
							  Xi'an, China
add solution to Exercise 4.3.

Included is the following file:

	pie.py

Thank you!
							Lawrence X. Amlord
							  Apr 13th, 2014
							  Xi'an, China