exp/cookiejar: infrastructure for upcoming implementation

This CL is the first of a handful of CLs which will provide
the implementation of cookiejar. It contains several helper
functions and the skeleton of Cookies and SetCookies.

Proper host name handling requires the ToASCII transformation
from package idna which currently lives in the go.net
subrepo. This CL thus contains just a TODO for this issue.

R=nigeltao, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7287046

Committer: Nigel Tao <nigeltao@golang.org>