Add user controller with #show and #new (signup)

Resolves #11. Add a basic form that allows user creation. No sessions yet.
This is a largely personal preference sort of thing, but I tend to reserve the `describe '#something'` naming scheme for instance methods in unit tests. For integration specs I typically like names that are descriptive of users' actions:

```ruby
describe "a user on the sign up page" do
  before do
    visit '/'
    click_on 'Sign Up'
  end

  context "signing up with no invalid input" do
    # ... yadayada
  end
end
```