Contributions in rendering, php-bibtex

- introduced LAYOUT option (values='table', 'list' or 'deflist') to allow different html structures (table+tr, ol+li, div+dl, respectively). The option "list" is only compatible with "index", since only monotonic numbers can be used with html lists.

- introduced Q_KEYS options (value=json-encoded array of bibtex entries) to limit the search to a list of bibtex entries

- introduced Q_ASSOCKEYS option (value=boolean) to interpret Q_KEYS as an associative array (or not, if set to false), then the keys in Q_KEYS are used to set the entries' Abbrv.

- introduced bibtexbrowser.bibliography.php and Q_BIBLIOGRAPHY option (value=boolean) to get bibtex references in html/php. One can use this by including bibtexbrowser.bibliography.php at the beginning of a page, then citing in the html body using <?php cite("MyBibtexKey1","MyBibtexKey2") ?> and finally calling "make_bibliography()" where the references should be printed (must occur AFTER all citations; unlike latex+bibtex, we do only one pass).

- introduced ICONS constant: a json-encoded associative array to provide icon urls for bibtex fields (eg: url, pdf, bib, gsc, etc...). Also implemented functions to read them out and render the corresponding html code.