google: Update godocs to reflect recent GCP front-end changes.
google: Update godocs to reflect recent GCP front-end changes.

Updated references to the API manager to reflect the new stand-alone API
manager at http://console.developers.google.com.

Change-Id: I156cce7b844850a560bfbf2b64fb0d8da5edaefe
Reviewed-on: https://go-review.googlesource.com/22706
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

drewwells/oauth2,golang/oauth2