net/http: add Transport.DialTLS hook

Per discussions out of https://codereview.appspot.com/128930043/
and golang-nuts threads and with agl.

Fixes Issue 8522

LGTM=agl, adg
R=agl, c, adg
CC=c, golang-codereviews
https://codereview.appspot.com/137940043
doc: add a few http items to go1.4.txt

CC=golang-codereviews
https://codereview.appspot.com/132670045