Anonymous sections

Rather often, I find myself in need of a namespace for elements - something that quacks like a section, but doesn't require an explicit class declaration. Anonymous sections to the rescue!

```ruby
class HomePage < SitePrism::Page
  section :header, '.header' do
    element :title, 'h1'
  end
end
```

The same thing works with the `sections` method.

```ruby
class HomePage < SitePrism::Page
  sections :articles, '.article' do
    element :title, 'h2'
    element :content, '.content'
  end
end
```