(Rebased) Pr/behind proxy middleware

[xsawyerx]

This is a rebased branch of #590. :)

It's only for review against the original PR. Please do not merge it.

[veryrusty]

I still think this is a better way to handle the psgi env munging needed (compared with including it as part of our Request class).

It's taken some time, but I concur with the comments on the original PR about the namespace; something like Plack::Middleware::Dancer2BehindProxy would be better (and hidden from pause?)