Changing the layout links: Add 'Sign out' & 'Profile' links for signed-in users

Several techniques
1. If-else branching in ERb (and the signed_in? helper method)
2. HTTP DELETE to 'signout_path' using '<%= link_to "Sign out", signout_path, method: "delete" %>'
3. Drop down menu using Bootstrap, see samples at http://getbootstrap.com/2.3.2/components.html
Changing the layout links: Include Bootstrap JavaScript library using
1. The asset pipeline (manifest file application.js)
2. Sprockets directives (e.g. //=require)
   for complete list see https://github.com/sstephenson/sprockets#sprockets-directives)
3. The presence of bootstrap-sass gem

The drop down menu was not working because the Bootstrap Javascript library was not loaded
Signin upon signup: Signing in the user upon signup (via the 'sign_in' helper method)
Sign out: A failing test for signing out a user (by checking the appearence of 'Sign in' link)
Sign out:  Destroying a session (user signout), responds to DELETE to /signout
Sign out: Add 'sign_out' method, it
1. Update remember_token in database, so the token in cookie could no longer authorize user
2. Tell user browser to delete the remember_token in cookie
3. Set self.current_user to nil, in case 'sign_out' method is called without a redirect (remember a new http request will cause @current_user to be nil?)