[BUGFIX beta] Update backburner.js.

Update with fixes for IE8 by @tricknotes (THANKS!).
Update jQuery version in tests.
[DOC beta] Fix default value of RenderBuffer's props
[BUGFIX beta] Normalize HTML string for IE8

This fixes are specified for IE8.

* Force HTML tag to lowercase.
* Add several `class` to be quoted
* Trim newline at the beggining of HTML tag
[DOC release] Documents Application.initializer