Allow options and custom metadata in multi-part uploads

I added an "options" param to s3 Multi and InitMulti so that users can add custom metadata to multipart uploads. We're running this on our test server now, and have confirmed that the metadata is being saved to s3 multi-part uploads.

I added some tests as well, and those are passing.

Note that this does change the API for bucket.Multi and bucket.InitMulti, but it makes it consistent with the params used in bucket.Put.