* Add cookiejar types
At current, it is not existent. It is used by the `superagent` library
but also not being typed therefore omitted.
It's rather lazy adaptation, with copy and pasted documentation and
tests.
A commit will follow, to extend the `superagent` library.
* Extend agent with cookie jar
Now that there are types for `cookiejar` library, we can actually make
sure it also exists here as it is used in the original JS libary[1].
[1]: https://github.com/visionmedia/superagent/blob/master/lib/node/agent.js#L31
