Make nbconvert html full output like notebook's html.

Basically makes the DOM map one to one.
I used a custom in-house HTML diffing rig - https://gist.github.com/jdfreder/9241376
Removed input div from markdown and heading cells
Removed some box-flex1 classes
Added output_pyerr to python error output
Get rid of non-existant display data tag
also add output_javascript
Added output_subarea class to rendered_html
Added output specific ouput_* classes
Fixed classed for latex and text output
Added readme in nbconvert tests dir.
Moved CSS into <style>
Remove 'unselected'
Audit of putput_* in nbconvert
still need to do notebook...
Removed notebook specific divs,
Fix for double div on pyout.
Add extra_class arg to the rest of the append methods,
use new extra_arg to add output_pyout class
CDN JS resources
Fixed rebase issues
addClass instead of adding extra_class arg everywhere
s/extra_classes/extra_class
https
Fix blank cell collapsing.