Set ElasticSearch similarity measures pr property

[stain]

Some fields are more important than others (e.g. "title") - and should get higher ranking in the results.

A quick method for that is to allow "mapping" to be set per property in the populator config, and then just populate the elasticsearch mapping directly with that configuration.

Relevant links:
http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/changing-similarities.html
http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/custom-analyzers.html
http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/query-scoring.html