Update MapReduce code to support Search as Input

[peschkaj]

MR can support search via

    {
     "inputs": {
                "bucket":"mybucket",
                "query":"foo OR bar"
               },
     "query":...
    }```

or

    {
      "inputs": {
                 "bucket":"mybucket",
                 "query":"foo OR bar",
                 "filter":"field2:baz"
                },
      "query":...
    }

[peschkaj]

Implemented in RiakBucketSearchInput class.