relationships_controller_spec.rb: added tests from Hartl listing 11.37
relationships_controller.rb: modified actions to respond to Ajax requests
app/views/relationships/create.js.erb, &/destroy.js.erb: created, added content from Hartl listings 11.39 and 11.40
user_spec.rb: added test from Hartl listing 11.41
user.rb: modified 'feed' method to have content of Hartl listing 11.42
micropost.rb: added 'from_users_followed_by' method from Hartl listing 11.45