Add cookie support to Pest

[torenware]

I've implemented a way of adding a set of cookie key/value pairs to a Pest request. It works like Pest::setupAuth() and Pest::setupProxy(): it turns the key value pairs into something that can be passed to curl_setopt().

This works at least well enough so that I can authenticate using session cookies against Drupal 7; I expect that most other apps that do this will work as well.

I'll be setting up a pull request shortly.

Thanks,
Rob