Google Docs Download(s)

Google Doc files, in the [drive.File](https://godoc.org/google.golang.org/api/drive/v2#File) entity, have an empty DownloadUrl field as they are not stored in Drive. However, they do have an ExportLinks<mime> map field which has a similar url to download and export Google Doc files. [[source](https://developers.google.com/drive/web/manage-downloads#downloading_google_documents)]

This patch assigns the ExportLinks<mime> field if the file url was empty after assigning the DownloadUrl to it.

Should resolve google/skicka#26.

Inadvertently, [vim-go](/fatih/vim-go) ran gofmt on the project when I exited the file and the end result was simply the re-alphabetization of the imports.