Minor improvements about helm-bibtexkey-parser

- Rename from helm-bibtexkey-parser to helm-bibtexkey-parser-program

    According to Elisp variable name conventions. for more information,
    See http://www.gnu.org/software/emacs/manual/html_node/elisp/Tips-for-Defining.html#Tips-for-Defining.

- Change default value

    Set default path of parser program in the same directory as helm-bibtexkey.el.
Merge pull request #1 from yasuyk/fix-helm-bibtexkey-parser

Minor improvements about helm-bibtexkey-parser