Added DOMParser parseFromString shim for text/html

As mentioned in issue #168 there are problems when using DOMParser.parseFromString with mime type 'text/html' on certain browsers e.g. current Opera 12.12.

MDN shows how to use some lines of shim code to support parsing of 'text/html' content (https://developer.mozilla.org/en-US/docs/DOM/DOMParser#DOMParser_HTML_extension_for_other_browsers).

With this code called from inside initializeTurbolinks, turbolinks works for me with Opera 12.12.