H1: send confirmation email: basics

Unlike the book, we had to manually delete the file

  app/views/order_notifier/received.html.erb

because its existence made Rails automatically choose it over the
`received.text.erb` file we created to send plaintext emails. This
`received.html.erb` file was auto-generated by the `rails generate
mailer` command.

Also, although the mailing mechanism has been tested, we have not
commited the username and password of the smtp settings in

  config/environments/development.rb

for security reasons.
H1: send confirmation email: html email and testing

The two tests in `order_notifier_test.rb` fail; the book does not
explain how the orders are generated. Presumably, we can fix this later.
(NOTE: The actual webapp does generate a proper email; it's only that
the test environment does not generate orders correctly).
H2: add integration test
I1: add users: scaffolding
I1: add users: bcrypt and username validation
I1: add users: test user creation (validation and redirection)
I2: authenticate users: generate controller
I2: authenticate users: administrative interface
I3: limit access
I4: add sidebar, more administration