Integrate resolvers into welle.kv/store and /fetch

Technically we don't need to use the riak.client.cap.ConflictResolver interface,
since it represents a pure function. However, sticking to Riak Java client's
public API here seems like a good idea:

 * New provided resolvers may be introduced in the future
 * More APIs may accept resolvers
 * It's easier to compare the code

Finally, it does not introduce any meaningful amount of overhead to use
resolvers instead of just functions. So lets give it a shot.

References #7
Mention conflict resolvers in change log