jwt: add JWT authenticated transport and client.

Self-refreshing OAuth transport and client needed by the
google-api-go-client services (i.e. to provide it out of
the box the same way as oauth.Transport, but for service
accounts).

LGTM=adg
R=golang-codereviews, gobot, adg
CC=golang-dev
https://codereview.appspot.com/42930045

Committer: Andrew Gerrand <adg@golang.org>
1 file changed