Ring Resizing Support

support for https://github.com/basho/riak_core/pull/301

This PR includes the following changes:

* exports the already existing `riak_kv_vnode:object_info/1` function, which is used during resize transfers
* add `riak_kv_vnode:request_hash/1`. has the effect of only forwarding puts/deletes during ring resizing, all other requests are handled locally
* slight modifications to AAE so entropy_manager and vnodes don't crash when ring is expanding
* modifications to PR/PW in get/put_core (see updated comments in source)