Updated bibtex completion

* Fixed a bug when BibTeX search did not find any bib files
* Added support for completion for 'thebibliography' environments,
  at least in the main tex file.  That is, this does not currently
  work for 'thebibliography' environments in inputted or included
  files.
* Only show 20 characters of author in popup window.  I have some
  bib entries with very many authors, and so sometimes the title
  was not displayed.  This is fixed now.
* Cleaned up syntax