Add within method for sections to allow block syntax in test code

This makes for shorter test code when working within nested sections.
 - similar to Capybara's within

Compare:
```ruby
  @page.top_section.child_section.should have_element1
  @page.top_section.child_section.should have_element2
  @page.top_section.child_section.should have_element3
````

```ruby
  @page.top_section.child_section.within do |child|
    child.should have_element1
    child.should have_element2
    child.should have_element3
end
````