wsgi: Support optional headers w/ "100 Continue" responses

Since WSGI is all about a single response to a single
request, in order for servers to process a PUT as a
single HTTP 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>