Removed 'find' exercise #4 and Bonus Challenge

these are both too difficult to do, even with extensive study of the 'man' page for 'find'
clarifying need for 'find' in Exercises
removed #4 'find' exercise and Bonus Challenge

these are not intuitive
clarified 'find' exercise descriptions
Fix control-d markup
Merge pull request #55 from njvack/2014-08-04-Davidson

Fix control-d markup in "Let the computer do the work"