Template that displays when the user is logged in.

Adjust the login test to check for the text in this template.

Create a couple new methods to extract login functionality and remove
duplicate test code.

When logging in with unrecognised credentials, the response should
include "Unrecognized user".

Don't wrap our routes/controllers in a `Sinatra::Base` class to keep
things simple. Was doing this because I thought I needed to in order to
be able to tell Capybara where the application was but looking at the
Sinatra testing page (http://www.sinatrarb.com/testing.html), they
provide an example for Capybara that details this
`Sinatra::Application.new` call that I can make instead. This frees us
up to use the simplified application definition.

Saw this on the Sinatra testing page
(http://www.sinatrarb.com/testing.html). Seems like I should put it in
to be a good Rack citizen.

* Write a `setup` method to extract the '/' route visit
* Change the logic of `test_index_has_login_form` to check for the
  presence of 'Login' text and a form (super basic)
* Create a new test that fills in the login form and clicks the "Login"
  button. For now just checks that we get a successful response.

* Add Rakefile to run tests with `rake`
* Add a `test_helper` to take care of the setup that Capybara requires
  in order to run
* Add a super basic test to see if Capybara works
* Put app in a `Sinatra::Base` subclass so that we can tell Capybara
  where to find it