Introduce the http-cookie gem.

HTTP::Cookie is prepended with a compatibility module and
Mechanize::Cookie is now an alias for HTTP::Cookie.

Mechanize::CookieJar now inherits from HTTP::CookieJar, adding
backward compatibility.
Fix tests that have become to fail.
Migrate the cookie API in the library part.
Reduce dependency on Mechanize::Cookie/CookieJar in certain parts.