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)').
I like your first suggestion, it's quite nice actually.

The second not so much, because in the default cause (all `True`), this will be a noisy return.