immutable: refactor uploader to do just encoding-and-uploading, not encryption

immutable: refactor uploader to do just encoding-and-uploading, not encryption
This makes Uploader take an EncryptedUploadable object instead of an Uploadable object.  I also changed it to return a verify cap instead of a tuple of the bits of data that one finds in a verify cap.
This will facilitate hooking together an Uploader and a Downloader to make a Repairer.
Also move offloaded.py into src/allmydata/immutable/.