Implement wsgi.file_wrapper support in WebOb

[digitalresistor]

Side note, it probably wouldn't be a bad idea to add https://github.com/Pylons/pyramid/blob/85900228894814105aa882e1e71f1e4e69090a9e/pyramid/response.py#L32-L77 to webob which supports wsgi.file_wrapper which can use sendfile on conforming wsgi servers to avoid even reading the data in the worker threads.

It only supports the extension if you give it the request environ, but in theory that's solvable with better integration which can override __call__(self, environ, start_response) and grab it at render time.

Originally posted by @mmerickel in #360 (comment)