Basic Cookie and Set-Cookie headers implementation. (WIP)

This piece of code is not finished yet because i have some questions:

* How i need to format Set-Cookie header? I have some troubles because it's multiline nature: one line per cookie.
* There are nice library to work with cookies: https://github.com/alexcrichton/cookie-rs. We can use `Cookie` type from that library to parse `Cookie` header, but (1) the library has several dependencies and i'm not sure that we want to deal with them and (2) maybe someone wants to use another cookie library. I can leave the `Cookie` header as-is, use cookie-rs or make some more deep parsing to simple key-value pairs.