Escape quotes in HTML attributes

Quotes in HTML attributes are not quoted, thereby causing the HTMLParser to ignore the remaining part of the document. This bug was found due to a [Python-Markdown parsing bug I've filed here](https://github.com/waylan/Python-Markdown/pull/275).

The bug only happens when more than one filter that uses HTMLParser is chained. The input HTML is `<img ... title="&quot;title" />` and after one round of parsing it becomes `<img ... title=""title" />` and this input to the next filter will result in (silent) truncation due to invalid HTML.