Rename the directory for exercises in the textbook and modify .gitignore.

I've replaced the name of a directory 'Solutions_to_Exercises' with 'Exercise_in_the_
Textbook' in order to make it more distinguishable from other analagous directories in
the future.

The modification in .gitignore makes Git not to track a directory named 'Exercises_from
_Instructor's_Manual' additionally.

Thank you!
							Lawrence R. Amlord
							   Apr. 7, 2013
							   Xi'an, China