Provide a callback for certificate validation

Call the user if they want to in order to check the certificate/host info. Always call but provide our decision in the HTTP case. The user gets the last word if the callback is set.

- [x] Implement for HTTP
  - [x] Tests for HTTP
- [x] Implement for SSH
  - [x] Tests for SSH
- [x] Implement for WinHTTP
  - [x] Tests for WinHTTP
From the copy on the .com page, it sounds like paying customers get their own queue. No idea about paying for something that's open. The OSX queue is bound to remain the longest though, as that requires actual hardware.

There's another thing we could cut down on: we run valgrind on every single (linux) job, which we don't necessarily have to. We could do it once per build and still be well-covered. When I look at the output, I use it more as a way to check whether I should run it locally and dwelve deeper, rather than as the real source of data (again due to the lag).

We initially put valgrind in there to help @nulltoken and his non-unixy ways, but IIRC we had far fewer builds then. Would you be ok with reducing the valgrind runs to once per build rather than on each job?

> Now - parallelising clar would be an interesting task.

Yes, yes it would. We have so much IO that it should be very noticeable, even on a single core.