import downloader complete (untested)

Made some api changes.

listImportableContents needs to provide the size
of the data, so the downloader can check disk free space.

retrieveExportWithContentIdentifier is passed the filepath to write to

Use temporary "CID" key during download of a ContentIdentifier from a
remote, so withTmp can be used and then move the content to the real key
once it's known.

Remote/Helper/ExportImport.hs

@@ -53,7 +53,7 @@ instance HasImportUnsupported (RemoteConfig -> RemoteGitConfig -> Annex Bool) wh
 instance HasImportUnsupported (ImportActions Annex) where
 	importUnsupported = ImportActions
 		{ listImportableContents = return Nothing
-		, retrieveExportWithContentIdentifier = \_ _ _ _ -> return Nothing
+		, retrieveExportWithContentIdentifier = \_ _ _ _ _ -> return Nothing
 		, storeExportWithContentIdentifier = \_ _ _ _ _ -> return Nothing
 		}