Fix for #6: Use tokeninfo API for Bearer token validation in dev_appserver

`auth.go`, `auth_test.go` and `context.go` changes are already in `t2-auth` (CL #12). I stupidly rebased `t6-tokeninfo` into `t2-auth` locally before realizing I pushed all the changes to the upstream. Sorry about that.

Anyway, this change goes together with the latest from `t2-auth`
(#12 - Store http.Request reference in endpoints.Context).

I'm not going to pull anything for real. Just wanted to keep references between branches and issues.