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 the suggestion. If you go that route, does that pattern start to become more a part of the library (i.e. registered callbacks)? If no, is it bad to have this single instance as a standalone?