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.
I agree, I actually initially had it in pull.go. However, remember that we don't wanna clobber Google Docs on the cloud with their empty replacements on disk.