entries2html.xsl: excerpt needs to be escaped for json output

[kswedberg]

I did a quick and dirty transform so we would have something for the excerpt field:
https://github.com/jquery/api.jquery.com/blob/master/entries2html.xsl#L107-115

However, it doesn't get everything from the <desc> node as it should. Instead, it stops when it encounters a tag inside of it.

My xsl-fu is not strong enough to figure out how to do this, but basically we need to import the xml2json template and then apply it to the contents of that <desc> node (which is the value of the excerpt prop.