Refactor storage interface to return io.Reader and Writer instances

Doing so will enable encrypting/decrypting files in a 'pipeline' without
having to write the intermediate results to a file or store them in
their entirety in memory.

This commit also updates the existing storage classes (local and FTP) to
meet the new interface definition.

asink/storage.go

@@ -7,11 +7,14 @@ package main
 import (
 	"code.google.com/p/goconf/conf"
 	"errors"
+	"io"
 )
 
 type Storage interface {
-	Put(filename string, hash string) error
-	Get(filename string, hash string) error
+	// Close() MUST be called on the returned io.WriteCloser
+	Put(hash string) (io.WriteCloser, error)
+	// Close() MUST be called on the returned io.ReadCloser
+	Get(hash string) (io.ReadCloser, error)
 }
 
 func GetStorage(config *conf.ConfigFile) (Storage, error) {