Support optional headers w/ "100 Continue" responses

WSGI allows only a single response to a single request.  In
order for servers to process a PUT as a single request and
at the same time pass **hints** back to the client (for
example, a payload type that the server can accept, or to
inform the client that the server expects to receive payload
metadata at the end of the payload transfer, etc),
"100 Continue" response headers is pretty much only place to
add these hints.

This is consistent with **RFC2616, section 10.1**:

10.1 Informational 1xx

This class of status code indicates a provisional response,
consisting only of the **Status-Line** and **optional
headers**, and is terminated by an empty line.

Openstack Swift has an immediate use case for this feature
where the object server will use an
"Accept-Payload-Footer: 1" like header to hint the proxy
server to send a payload footer after it has finished
transmitting the payload (for sending metadata for an
erasure coded or encrypted payload, enforcing end-to-end
Etag checks, etc).

Addresses eventlet enhancement #126

Signed-off-by: Tushar Gohad <tushar.gohad@intel.com>