Fixes #297 Stackoverflow oauth2 endpoints constant added. Ref: https://api.stackexchange.com/docs/authentication Change-Id: I8285a9baa9514c79e8325c40434f62aa1ed4939b GitHub-Last-Rev: 909997b81e544c02130cc08f5fc1cea65872a7a0 GitHub-Pull-Request: golang/oauth2#298 Reviewed-on: https://go-review.googlesource.com/119995 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>