Clean up documentation for element classes
Add Image#to_html
Use the top-level <math> element for MathML

Now, Subpod#mathml returns a MathML object initialized with the correct
top-level <math> element, not Wolfram Alpha's <mathml> wrapper.
Add Element#inspect

Before, Element#inspect returned a massive string showing the complete
hierarchy for the Nokogiri::XML::Document instance.

Now, Element#inspect simply returns the element's xml.