Fix grammar

Change 'You want to create a class methods and instance methods'
to 'You want to create class and instance methods'
Merge pull request #112 from boatrite/grammar-fix-class-methods-chapter

Fix grammar
typo/incorrect word usage

reference definition http://www.oxforddictionaries.com/us/definition/american_english/overwrite
Merge pull request #114 from galenandrew/master

typo/incorrect word usage
Merge with upstream