Refactor term vector api

This is necessary to allow adding a mult term vector request
Multi term vector request
--------------------------

This feature allows to retrieve [term vectors](https://github.com/elasticsearch/elasticsearch/issues/3114) for a list of documents. The json request has exactly the same [format](https://github.com/elasticsearch/elasticsearch/issues/3484) as the ```_termvectors``` endpoint

It use it, call

```
curl -XGET 'http://localhost:9200/index/type/_mtermvectors' -d '{
    "fields": [
        "field1",
        "field2",
        ...
    ],
    "ids": [
        "docId1",
        "docId2",
        ...
    ],
    "offsets": false|true,
    "payloads": false|true,
    "positions": false|true,
    "term_statistics": false|true,
    "field_statistics": false|true
}'

```

The return format is an array, each entry of which conatins the term vector response for one document:

```
{
   "docs": [
      {
         "_index": "index",
         "_type": "type",
         "_id": "docId1",
         "_version": 1,
         "exists": true,
         "term_vectors": {
         	...
         }
      },
      {
         "_index": "index",
         "_type": "type",
         "_id": "docId2",
         "_version": 1,
         "exists": true,
         "term_vectors": {
         ...
         }
      }
   ]
}
```

Note that, like term vectors, the mult term vectors request will silenty skip over documents that have no term vectors stored in the index and will simply return an empty response in this case.

Closes #3536