jQuery.parseHTML: Mitigate XSS vulnerability

Scripts passed in event attributes are executed in `parseHTML` immediately,
without any possibility for the user to intervene:

`jQuery.parseHTML('<img src=x onerror=alert(1)>');`

To mitigate this vulnerability `document.implementation.createHTMLDocument()`
should be used as standard context instead of `document`. Now the user has
to set a different context deliberately for this issue to occur.

See also GitHub issue #1505.

**EDIT:** Agreed to the CLA.
Also, please undo the jshint exception and move the ternary `?` and `:` to the end of the preceding lines (see http://contribute.jquery.org/style-guide/js/#multi-line-statements ).