storage: add default user agent and scopes.
storage: add default user agent and scopes.

Fixes #192

Change-Id: Ib1e5cbc091b65e4396aa003251cbd770d86e4a47
Reviewed-on: https://code-review.googlesource.com/3830
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Glenn Lewis <13a1c799ae0d2ab8fa6a3294896d9a3ccca89794@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go