Make 'Bucket.delete_keys' return a useful result.

Result is a list of (key, success) tuples, one per requested key.

Fixes #161.

Addresses #160 (users who want to delete without raising can use
'Bucket.delete_keys([key])' instead of 'Bucket.delete_key(key)').
A somewhat more flexible option would be to pass in an "on_error" callback (the caller who wanted a list could create it and pass its `append` method, for instance).