Support for changing id via script
I know two ways to change id.

1. The simply way:

 ```
 "script": "ctx.id = ctx.doc._id = ctx.doc.new_id;"
 ```

2. Associate the _id mapping with a path:

 ```
 {
     "mappings": {
         "my_type": {
             "_id": {"path": "new_id"},
         }
     }
 }
 ```

 and set id to null:
 ```
 "script": "ctx.id = ctx.doc._id = null;"
 ```

But in both cases a river doesn't change id keeping it always as the couchdb's id.

Closes #33.