Added swift3.Response

Swift normalizes response headers with Python's str.title().  However, there
are some S3 clients who don't accept such kind of headers.  For example, AWS
Java SDK expects that a etag header is 'ETag', not 'Etag'.

This patch introduce our own Response class, which inherits swob.Response.  The
main differences from the original class are:

 - Normalizes response headers as S3 clients expect.
 - Replaces Swift specific headers like 'x-object-meta' with S3's ones.

Change-Id: I4a4443fc0b75e75ea7e2e45eede43f12b040a7a3