Allow bib files to have a keyword including "-"

currently import fails on bibtex files with a keyword containing "-", e.g.
@TECHREPORT{cms:wh:ww,
  year = {2013},
  mendeley-tags = {cms,hww,vh},
}

This patch fixes the parser to allow dashes in keywords. The regexp could probably be improved to take anything but \s or =
Merge pull request #3 from robinro/master

Allow bib files to have a keyword including "-"