completed subsection 10.1.2
completed subsection 10.1.4
completed subsection 10.1.5
completed subsection 10.2.1
completed subsection 10.2.1
completed subsection 10.2.2
moved signed_in_user method to sessions_helper
completed subsection 10.3.2
completed subsection 10.3.3
completed subsection 10.3.4
completed chapter 10