Added support for passing-through client cert and key

[gzurbach]

I spent the last two days struggling to get a test using certificates working with node-replay.

The following snippet shows where my test was failing:

var options = {
  key: certificate.privateKey,
  cert: certificate.certificate,
  // more stuff
}

var request = https.request(options, function(response) { 
  // getting a 403 with node-replay, but 200 when running the app
});

The change in this PR allowed me to fix my test. The test runs with both node 0.10 and 4.4.

These options are listed here: https://nodejs.org/api/https.html#https_https_request_options_callback (see the last example at the bottom of the page).

Please let me know if I am using the wrong approach. Great library btw! :)