Merge pull request #339 from tseaver/get_metadata-map_accessors

Make KeyError from 'get_metadata' more flexible.
Add location support to buckets.

See: https://cloud.google.com/storage/docs/json_api/v1/buckets and
https://cloud.google.com/storage/docs/concepts-techniques#specifyinglocations

Addresses 'location' part of 314.