Consider using boto3 s3 transfer interface

[ericmjonas]

http://boto3.readthedocs.io/en/latest/_modules/boto3/s3/transfer.html

[shivaram]

This will be a part of the discussion in #108

[ooq]

So the transfer APIs are only for files. I think the only place we can use it now is in wrenhandler.py. I'll change the wrenhandler to use transfer API.