twisted.web.proxy should send (x-)forwarded-for header

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This is a long-standing convention that is at least partway through the standards process:  http://tools.ietf.org/html/draft-petersson-forwarded-for-02

All outgoing HTTP requests from twisted.web.proxy.ProxyClient should (optionally) emit a Forwarded-For and X-Forwarded-For header that describes the original client that made the request that they are forwarding. This should be easily configurable at the level of instantiating ReverseProxyResource and Proxy.

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I need to fill this out with some more details; if you want to implement it please comment first so I can fill you in.

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I think that this is all there is to say about it, but again, please feel free to ask for clarification if there is not enough specificity.