enhance ex20-editor with synchronization in Lecture 20: Queues & Locks.

Augmented are the following 3 files:

	Lectures/L20-Queues_&_Locks/code/ex20-editor/src/editor/EditBuffer.java
	Lectures/L20-Queues_&_Locks/code/ex20-editor/src/editor/GapBuffer.java
	Lectures/L20-Queues_&_Locks/code/ex20-editor/src/editor/SimpleBuffer.java

Thank you!
							Lawrence X. Amlord
							  Nov. 25th, 2014
							   Xi'an, China