exp/cookiejar: implement Cookies and provided tests

This CL provides the implementation of Cookies and
the complete test suite. Several tests have been ported
from the Chromium project as a cross check.

R=nigeltao, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7311073

Committer: Nigel Tao <nigeltao@golang.org>