feat(falcon.Response): Support wsgi.file_wrapper

This patch adds support for wsgi.file_wrapper, if it is provided by the
WSGI server, to more efficiently pipe resp.stream to the client.

NOTE: Applications do not need to do anything differently; Falcon will
automatically use wsgi.file_wrapper if it is available, and a file-like
object has been assigned to the resp.stream attribute.

See also PEP-0333: http://goo.gl/Y1pbQD

Closes #181