Use HTML to parse remote pages in Nokogiri

Use `to_xml` for pretty prenting
Model style tweaks
Remove extra mark-up in stand-alone version
Add support for iframes

Usage:

Set `iframe: true` in model's YAML Frontmatter to embed that model in iframe.
Useful for 'leaky' stylesheets (e.g. Bootstrap or external imports) that are difficult
to scope under module (include body and other default styles that may affect Glypotheque.

Additionally use `pretty: true` on the model to force reformat HTML remotely fetched HTML
Always render Slim templates 'pretty'
Refactor and improvements
Update README.md
Move iframe resize function into the JS file
Use pry instead of pry-plus
Merge pull request #2 from tyom/iframe

iframe support