exp/cookiejar: implementation of SetCookies

This CL provides the rest of the SetCookies code as well as
some test infrastructure which will be used to test also
the Cookies method. This test infrastructure is optimized
for readability and tries to make it easy to review table
driven test cases.

Tests for all the different corner cases of SetCookies
will be provided in a separate CL.

R=nigeltao, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7306054

Committer: Nigel Tao <nigeltao@golang.org>