Fix chapter 3 typos
Merge pull request #5 from humboldtux/typos

Fix chapter 3 typos
more tweaks
Merge branch 'master' of github.com:GoBootcamp/book

* 'master' of github.com:GoBootcamp/book:
  Fix chapter 3 typos
reorganized the content + added a new section on composition
copy edit
copy edit
typo fix
removed unused pics and added cover in jpg
copy edit to answer Loren's question
edits around mutability
cross reference
broke down methods and interfaces
more cleanup
changed the map exercise to be less geeky
added a switch case section
added more info about type assertion