Stream the s3 object to the client directly

Instead of reading the bytes back from s3, send the stream as part of
the response to be handled by the HTTP server directly. It turns out
that http-kit handles this case well in that it will close the stream
after the bytes have been read in.
Fix the unit tests.
Merge pull request #14 from Wikia/stream-directly

Stream the s3 object to the client directly