Add new ACL functions.

These new functions operate on Buckets, Default Objects, and individual Objects.
The API doesn't contain any terminology that would evoke EntityRole. I have done a bad job with AccessControlRules too. What about

PutDefaultACLRule
PutACLRule
etc.

ACL for plural, ACLRule for singular. We can also rename AccessControlRule to ACLRule, which would make more sense to the user and is shorter.

How does this sound?