Move put merge code inside riak_object

Since the put_merge code was copied into two tests, both of which
only manipulate riak_object, it seemed a refactor made sense.

Looking at the put merge code, it only manipulates internal state
of riak_object, so I think it is the correct place for the code.

It also smooths the way for a general refactor of riak_object
into a CRDT and riak_kv into riak_crdt (mwah ha ha ha ha!)
Slight doc / comment clean up