Got S3 import and export working fully for both versioned and unversioned
buckets. This included developing a patch to the aws library; only
versioned buckets are fully supported until that gets merged.

I'm left with one blocking problem before merging `import-from-s3`: The commit
history when importing from a versioned bucket is too long. It needs to
find the point in the versioned import that has already been committed and
avoid committing it again. Have started on that, but didn't get all the way
today.

Also, this S3 import feature should be able to be used with anonymous S3
access to a bucket, and indeed that might be more common than wanting to
import from a bucket you own or have credentials to allow access to. 
But the S3 remote does not currently try to use anonymous S3 access,
so supporting that will need some more changes.

(Keyboard is fixed, yay!)
