`Node.toggleView` did not show Node when `style="display: none"`
`node-view`'s `_isHidden()` relied on `hidden` attribute,
now also checks and `display` style (in line with `_hide` and `_show`)
`node-view` requires `dom-style`
Improved node-view's _isHidden
See PR 895
`node-view` `_hide` sets `hidden` attribute to empty string
That's what the spec says! See
* http://dev.w3.org/html5/spec-preview/common-microsyntaxes.html#boolean-attributes
* http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-F68F082
Adjusted node-view tests to proper use of hidden attribute
Don't break the back/forward cache. Fixes #959

Attaching an unload listener breaks the back/forward cache in modern
browsers, and is only necessary in old versions of IE that can leak
memory across pageviews if events aren't detached.
After node-pluginhost mixes PluginHost into Node, update all Node._instances
Added tests for node-pluginhost tweak
(see previous commit)
ButtonGroup.disable disables all child buttons / inputs
See #973
Tree.Sortable: Reindex children after sorting.
Merge branch 'tree-sort-reindex' of git://github.com/rgrove/yui3 into dev-master

Conflicts:
	src/tree/HISTORY.md
	src/tree/tests/unit/assets/tree-sortable-test.js
Build tree.
Merge branch 'node-pluginhost-instances' of git://github.com/asystance/yui3 into dev-master
Build node.
Update Node history.
Merge branch 'node-view-displayStyle' of git://github.com/asystance/yui3 into dev-master
Build node.
Update Node history.
Merge branch 'dev-master' into dev-3.x
Merge branch 'event-bfcache' of git://github.com/rgrove/yui3 into dev-3.x