acme: default values and discovery

Add Directory to the Client fields. This removes ACME resource URL
argument from almost all of the Client methods and makes it easier to
use.

Change-Id: I245c975fda914d50fe62ebbec55a4fcf34cb57f2
Reviewed-on: https://go-review.googlesource.com/23972
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed