Update to work for Rails 4 + small edits

Quick review of sample app workflow plus some changes with how Rails 4 deals with the default `public/index.html` page.
Clarify that they *should* create an account now.
Fix some typos.
Use rails generators to create controller.
Move the login/logout links closer to the end.
Prefer code over strings.
Specify that this is a terminal command.
Merge branch 'edits-cleanup' of github.com:nuclearsandwich/railsgirlsrdu into nuclearsandwich-edits-cleanup