Additional SchemeHandlerResponse options

This PR adds a number of properties/options to SchemeHandlerResponse:

* StatusCode, to return something else than status 200 to CEF.
* RedirectUrl, to have CEF do redirects.
* ContentLength, to provide a content length to CEF regardless of whether the response stream is seekable or not, for optimal response reading.
* CloseStream, so that SchemeHandlerWrapper can close the response stream once it has read all data.

It also fixes a bug in SchemeHandlerWrapper, which didn't return false upon response completion in ReadResponse.

The ultimate goal of all changes were to be able to write a SchemeHandler implementation for proxying HTTP requests, which now is possible.

I'm sorry to say that I couldn't write any unit tests. I don't know how to instantiate SchemeHandlerWrapper from the CefSharp.Test project. Any suggestions?