Fix #1424 -- add support for SSL client certificates

The test is a bit basic but follows what's being done for the `--cert`. I had test failures when running tox locally but the test I added passes.

I tried locally with a self-signed CA and it worked fine. It'd be perfect for [securing devpi instances](https://bitbucket.org/hpk42/devpi/issue/74/add-option-to-devpi-client-to-send-ssl) :)