Add sibling-resolution functions.

A resolver function takes a RiakObject and (hopefully) resolves its siblings into a single sibling. Resolvers are invoked automatically when fetching an object returns siblings.

The resolver can be set on the client, the bucket, or the individual object. If the object in conflict has no resolver, the bucket's resolver will be used, and then the client's resolver if the bucket has none.

The default resolver does no resolution. Also included (but not assigned) is a resolver that selects the latest sibling based on the `last_modified` property; see riak/resolver.py for more details.