Driver can enable or disable PhantomJS cookies

As of 1.7, PhantomJS allows its `CookieJar` to be enabled or disabled using the
`cookiesEnabled` function. This commit adds access to that from the driver
using a similarly named `cookies_enabled=` method.

For more information on the API, check out:

https://github.com/ariya/phantomjs/wiki/API-Reference-phantom#cookiesenabled-boolean