Add anonymous section capability to element method via the block

I have been working with a modified version of this gem, and added the functionality to define an anonymous section by passing a block to the element method.

This allows you to do something like:

``` ruby
element :anonymous_section, "div#anonymous_section" do |section|
  section.element :span_inside, "span"
end
.....
# Now you can reference the span inside the anonymous_section just as if you had created a separate AnonymousSection < SitePrism::Section class
anonymous_section.should have_span_inside
````