Add support bucket index [JIRA: CLIENTS-879] #622

Hi,
I would like to use $bucket secondary index using java riak client.
I have noticed that such possibility was added some time ago in: #129.
It seems that in current version we can't use $bucket index in Java client (I am using client ver. 2.1.4).
In documentation there is a note that we can use it via REST API: http://docs.basho.com/riak/kv/2.1.4/developing/usage/secondary-indexes/#retrieve-all-bucket-keys-via-the-bucket-index
but there is no example how to do it in java.
The old code, which was added in #129 look like follows:
final Bucket b = riakClient.fetchBucket(bucketName).execute(); all = b.fetchIndex(BucketIndex.index).withValue("_").execute();