Add ability to attach sections without explicitely set section's ClassName

Current realisation doesn't allow sections to be attached without explicit class name set, for example we are forced to use this:

```ruby
class LoginPage < SitePrism::Page
  section :top_bar, TopBar, '.top-bar'
end
```

when we can just use this:

```ruby
class LoginPage < SitePrism::Page
  section :top_bar, '.top-bar'
end
```

`TopBar` class name generates automatically with `camelize` method. This also doesn't break existing behaviour and generates class name from section name only when no explicit class name provided.

P. S. This also partially allows us not to force-loading sections before pages in `rails_helper.rb`, i. e. instead of:

```ruby
Dir[Rails.root.join('spec/support/{sections,**}/**/*.rb')].each { |f| require f }
```

we can just require like this:
```ruby
Dir[Rails.root.join('spec/support/**/*.rb')].each { |f| require f }
```

because now error `uninitialized constant LoginPage::TopBar` won't appear. But still if we wan't to use custom class name (different from `section_name.camelize`) this error will be thrown. Considering defining class name as a string? Or provide a handy way to require sections before pages.