dropbox: support Dropbox content hashing scheme - fixes #1302

* add support to hashing module
  * add dbhashsum to list the hashes
  * add support to dropbox module

This means objects up and downloaded to/from Dropbox will have their
hashes checked.

Note after this change local objects are calculating MD5, SHA1 and
DBHASH which is excessive and needs to be fixed.

fs/operations.go

@@ -978,6 +978,15 @@ func Sha1sum(f Fs, w io.Writer) error {
 	return hashLister(HashSHA1, f, w)
 }
 
+// DropboxHashSum list the Fs to the supplied writer
+//
+// Obeys includes and excludes
+//
+// Lists in parallel which may get them out of order
+func DropboxHashSum(f Fs, w io.Writer) error {
+	return hashLister(HashDropbox, f, w)
+}
+
 func hashLister(ht HashType, f Fs, w io.Writer) error {
 	return ListFn(f, func(o Object) {
 		Stats.Checking(o.Remote())