This change breaks jubarecommender compatibility. 
http://jubat.us/en/api_recommender.html
How do you deal with this modification? (break compatibility or keep compatibility)

This factory seems to assume nested parameter like following:
```
  (snipped converter rules)
  "method": "nearest_neighbor_recommender",
  "parameter" : {
    "method": "lsh",
    "parameter": {
      "hash_num" : 128
    },
    "unlearner": "lru",
    "unlearner_parameter": {
      "max_size" : 20
    }
  }
```
and this style confuse with classifier config. classifier doesn't accept nested `parameter` objects.

configuration of previous recommender version is:
```
  (snipped converter rules)
  "parameter" : {
    "hash_num" : 128
    },
  "method": "nearest_neighbor_recommender:lsh"
```