x/crypto/ssh/agent: add a client example and tweak package doc. Change-Id: I373fdbb6351d71b12fcfed31cf4b08975a443294 Reviewed-on: https://go-review.googlesource.com/19894 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Reviewed-by: Adam Langley <agl@golang.org> Run-TryBot: Adam Langley <agl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>