Add score_mode to CompletionSuggester

Support various score_modes in the CompletionSuggester.  Modes supported are max, min, total, and product.  This is useful when you have multiple suggestions with the same output but different inputs and weights.

```
curl -X PUT 'localhost:9200/music/song/1?refresh=true' -d '{
    "name" : "Nevermind",
    "suggest" : { 
        "input": [ "Nevermind" ],
        "output": "Nirvana - Nevermind",
        "payload" : { "artistId" : 2321 }, 
        "weight": 3
    }
}'

curl -X PUT 'localhost:9200/music/song/2?refresh=true' -d '{
    "name" : "Nevermind",
    "suggest" : { 
        "input": [ "Nirvana" ],
        "output": "Nirvana - Nevermind",
        "payload" : { "artistId" : 2321 }, 
        "weight": 2
    }
}'

curl -X POST 'localhost:9200/music/_suggest?pretty' -d '{
    "song-suggest" : {
        "text" : "n",
        "completion" : {
            "field" : "suggest", 
            "score_mode": "total"
        }                                             
    }
}'
{
  "_shards" : {
    "total" : 5,
    "successful" : 5,
    "failed" : 0
  },
  "song-suggest" : [ {
    "text" : "n",
    "offset" : 0,
    "length" : 4,
    "options" : [ {
      "text" : "Nirvana - Nevermind",
      "score" : 5.0, "payload" : {"artistId":2321}
    } ]
  } ]
}

```

Not sure this is the best approach but I wanted to put it out there for comments.