Add manifest files with checksums listing each file in each tar partition

@fdr Here's a squashed commit with all the changes.

I need to speak to you about how to handle downloading the manifest files. Adding the second upload wasn't too much of a pain but adding a second download looks tricky.
On Fri, Jun 13, 2014 at 9:26 AM, gsstark <notifications@github.com> wrote:

> In wal_e/worker/upload.py:
>
> > @@ -120,18 +125,19 @@ def standard_detail_message(prefix=''):
> >                      raise typ, value, tb
> >
> >              @retry(retry_with_count(log_volume_failures_on_error))
> > -            def put_file_helper():
> > -                tf.seek(0)
> > -                return self.blobstore.uri_put_file(self.creds, url, tf)
> > +            def put_file_helper(_creds, _url, _file):
>
> I needed to call it twice so I made the arguments explicit parameters
> instead of class variables.
>
Oh, duh.