Code review comment for lp:~mew/charm-helpers/fetchallthethings

I've fixed all of these issues, except (4). The command argument "dest_file" refers to the downloaded file, not the destination for extraction. I've renamed that to "dld_file" to make that a bit clearer.

I've also rearranged a bit, moving the archive handling code to the payload.archive module. fetch.archive.UrlArchiveFetchHandler is now renamed to fetch.archiveurl.ArchiveUrlFetchHandler.