acme/autocert: new high-level package for automatic cert management

Package autocert provides automatic access to certificates
from Let's Encrypt and any other ACME-based CA.

It is heavily based on the ideas from rsc.io/letsencrypt.

Change-Id: I62021452a918cd49093162f3d6c74e9d7f452fb8
Reviewed-on: https://go-review.googlesource.com/23970
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed