Support 'nested' type

Adds support for Elasticsearch's nested mapping type. See http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-nested-type.html

In the current versie Chewy doesn't work well with configuration for fields, which parent field is mapped as type: 'nested'. E.g. all defined fields will get the default type 'string', even if a type is specified in the index. The parent is incorrectly interpreted with the 'multi_field' semantics.

This pull request fixes this behavior.