Fix bug with missing downloadURL for docs/sheets

Google Docs + Sheets do not populate 'DownloadUrl', but
instead provide an map of exportLinks ie csv, docx, pptx,
txt etc. Simple heuristic to determine the url to export
as well as create the target extension to avoid clobbering
of the original file on a re-sync.
This PR addresses issues #11, #31. It also potentially addresses  #29 but the receipe will be fully complete once PR #32 or even better PR #39 gets merged.
Good question, see I am thinking about the case that a file has both exportLinks as well as BlobAt. In this case we would want to download the actual file. I guess exportLinks and BlobAt if are mutually exclusive, then we can stick to your suggestion.