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.
And if there isn't a class that matches?