Updated demos and validated/checked all files

* Updated HTML, CSS and JavaScript for all remaining demos — except pr-answer, which doesn't seem to be working.

* Added `'use strict';` to all .js files.

* Added JSHint options to all .js files.
 
* Moved classes (e.g. VideoPipe) into separate .js files.

* Validated all .js files with JSHint.

* Validated/checked all .html files with W3C Validator and [htmlhint.com](http://htmlhint.com).

As discussed with @juberti, intention is to merge these changes in one go.