acme: context-aware Client methods

This change adds a context to all exported methods of Client
which may perform network requests, to allow for easier control
over request timeouts and cancellation.

Change-Id: I635a4d7ad39a63ed9e6823b1af12fbb201c19647
Reviewed-on: https://go-review.googlesource.com/27091
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>