Update converters/bibtex2html to 1.89

Version 1.89, 08/01/2008
========================
o entry types and field names now typeset in lowercase in f_bib.html
o new option -unicode to use Unicode entities for some macros
  (such as ◯ for \bigcirc, etc.)
o new option -html-entities to use HTML entities for some macros
  such as \le, \approx, etc. (patch by Steven G. Johnson)
o new option -header to insert a header (similar to -footer)
  (patch by Steven G. Johnson)

(tonio)