Modified Cookie implementation to conform to RFC 6265.

- Cookies are now parsed on the client side;
- Changed cookies field in the HTTPResponse from Cookie[string] to CookieValueMap;
- CookieValueMap is now an alias to DictionaryList!(string, true) instead of a struct;
- [De]Serialization of Cookie moved from free function in the server to Cookie;
- Added http_cookie_client and http_cookie_server examples.