Implement BibTeX export format

Adds BibTeX format so that requested citation of URL is
returned in BibTeX format.

 - Added function zoteroExportRequest to handle export
    functionality of zotero translation-server
 - Added helper function scraperHelper to handle export
    in conjunction with native scraper
 - Added convertToBibtexAsync to convert output of
    zoteroWebRequest (renamed from zoteroRequest) to BibTeX

Bug: T1083
Change-Id: Ib5f7a178bb94082cb155172ec2165f55353ec174