[patch] to BibTeX export, with item.webpage, write url to howpublished field

[ehet...@gmail.com]

Hi all.

This patch was originally at [1], with the message:

| Using standard BibTeX styles, url will not show up. Generally it
| should be added to the howpublished field. Here is some code to do
| so (to be added to doExport() for bibtex exporter):

best,
Erik Hetzner

1. http://forums.zotero.org/discussion/2133/when-exporting-a-website-export-the-url-as-howpublished/