Updated examples after Rulu 2013

Updated the Sinatra TODO-list app to use Widget.
Added Hexp.from_nokogiri as an example (for now)
Added  DSL inclusion support

An object that implements to_hexp can be used inside a Hexp literal as if it's a node.
Such an object can now also include the Hexp module to make the object behave even
more like a node, providing :tag, :attributes, :children, :attr, :rewrite and :to_html.
Fix "grouped expression" warnings when running with -w
Add the node.class?('foo') predicate to quickly test for presence of a CSS class
Improve documentation, back to >90% YARD coverage