Move vclock inside riakobject

This is another "Went the wrong way" once you start playing with the API.

By having the vclock in the `RiakObject` it becomes as transparent as possible to the end user; Get something from Riak, modify it, store it back. Never have to muck with the vector clock.

After conflict resolution and the update in Update value, we re-inject the vclock in case the user didn't deal with it properly.

I've also annotated `RiakObject` to make that work seamlessly without any type checking / different code paths.