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
  - [ ] Tests for SSH
- [x] Implement for WinHTTP
  - [x] Tests for WinHTTP
The Travis documentation says we have "up to two cores" available. We could do three per that ancient rule of thumb.