Implement server side copies if possible - fixes #99

Add optional fs.Copier interface

Implemented for
  * swift
  * s3
  * drive
  * dropbox
  * google cloud storage

fs/fs.go

@@ -24,6 +24,7 @@ var (
 	fsRegistry []*FsInfo
 	// Error returned by NewFs if not found in config file
 	NotFoundInConfigFile = fmt.Errorf("Didn't find section in config file")
+	ErrorCantCopy        = fmt.Errorf("Can't copy object - incompatible remotes")
 )
 
 // Filesystem info
@@ -149,6 +150,19 @@ type Purger interface {
 	Purge() error
 }
 
+type Copier interface {
+	// Copy src to this remote using server side copy operations.
+	//
+	// This is stored with the remote path given
+	//
+	// It returns the destination Object and a possible error
+	//
+	// Will only be called if src.Fs().Name() == f.Name()
+	//
+	// If it isn't possible then return fs.ErrorCantCopy
+	Copy(src Object, remote string) (Object, error)
+}
+
 // An optional interface for error as to whether the operation should be retried
 //
 // This should be returned from Update or Put methods as required