Update computed properties docs to reflect the fact that
they can now be published & data bound from the outside.

Fixes #633.
Fixed typo
Update polyfill-next-selector docs for #637
Update layout-elements.md

`mode` is an attribute on a header panel, not an element
API docs: add rest of fire() method signature
Add Plunker preview buttons to all Creating Elements examples
Merge pull request #638 from arthurevans/docs-633

Update computed properties docs to reflect the fact that
Typo correction propery > property
Merge pull request #644 from xpe/patch-1

Correct small typo on layout-elements.md
Styling - address feedback
Merge pull request #643 from Polymer/polyfill-next-selector

Update polyfill-next-selector docs for #637
Prep for release
Cache bust js
Fix 404 on site (link to sinspect-screen.png)
Merge pull request #651 from harshadsabne/master

Fix 404 on site (link to sinspect-screen.png)
Merge branch 'master' of https://github.com/Polymer/docs
Use layout attributes on header-panel example
Fix incorrect attributes
Merge pull request #654 from robdodson/layout-guide-updates

Use layout attributes on header-panel example
Note vertical layout needs height