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.
This doesn't have to be in remote.go. It should be likely in pull.go.

And it should have be imported. isGoogleDoc or better isExportable?