Initial impl of unified bindings for static and Shiny

I've only tested this for static so far, not with Shiny.
This commit should be considered a starting point for
discussion, not a final proposal.
Prefer definition over defaults. Heh.
Make unified bindings actually work with Shiny
Remove _output from widgetOutput div
Require type:'output' on output bindings
Add html-widget html-widget-output classes to widgetOutput
Rename widget_div to widget_html
Merge pull request #5 from jcheng5/unify-bindings

Unify bindings