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.
Fix a wrong origin given.
Track http-cookie 1.0.0.pre9.
Merge branch 'master' into http-cookie
Follow the latest change in CookieJar#parse.
Depend on http-cookie 1.0.0.
Merge branch 'http-cookie'.  Closes #257.