Fine grained control over sizing

Allows $width, $height, and $sizingPolicy elements of a widget object
to work together to determine the size a widget should be rendered at
for any given situation.

Still TBD: How to opt out of the RStudio Viewer pane
Docs
Sizing improvements, and beginning of vignette
pass init_result to resize method
Rename sizePolicy to sizingPolicy, allow suppression of viewer pane

- Adding the suppress_viewer classname to a widget prevents it from
  rendering in the viewer pane
- Renamed sizePolicy to sizingPolicy
- Tweaks to exports and docs
- Remove some dead code
Provide width/height to initialize even if not standalone
widgetOutput should pass through width and height to widget_html
Allow widgetOutput to take an exemplar for x

This allows widgets in multi-widget packages to use widgetOutput
Use correct CSS class when exemplar is used for widgetOutput
Refactor getDependency to return *all* necessary dependencies
Merge pull request #6 from ramnathv/sizing

Sizing