Output (more) valid html

This does two things:

* Use a document fragment so Nokogiri doesn't litter the output with
  hundreds of open&close body tags.
* Drop the old style doctype in favor of html5.
Rescue case with no config file.

This will allow Showoff to run in cases without a config file.

Fixes #191
Generate static supplemental materials

* Generate a static HTML copy of the presentation
  * `showoff static`
* Generate a printable HTML presentation
  * `showoff static print`
* Generate supplemental material (such as exercises)
  * `showoff static supplemental exercises`

Fixes #183
Fix notes for current redcarpet
Merge pull request #194 from binford2k/ticket/183/static_generate_supplementals

Generate static supplemental materials
Merge pull request #195 from binford2k/fix/strikethrough_and_section_notes

Fix notes for current redcarpet
Merge pull request #192 from binford2k/ticket/191/rescue_no_config_file

Rescue case with no config file.
Merge pull request #190 from binford2k/ticket/189/valid_html

Output (more) valid html
Pushed a new version to correct notes problem

* Continuing on in the grand tradition of rapid point releases, this is
  a minor, yet important, errata fix. Current versions of RedCarpet did
  not work with both instructor notes and `:strikethrough` enabled. This
  release turns `:strikethrough` back off and a future update will rework
  the syntax for instructor notes.
* Enables static supplemental material generation
* Output cleaner HTML
* Allows running without a `showoff.json` config file again.