Add ignore_conflicts parameter to put_mapping

as stated in the elasticsearch documentation:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/indices-put-mapping.html

> When an existing mapping already exists under the given type, the two mapping definitions, the one already defined, and the new ones are merged. The `ignore_conflicts` parameters can be used to control if conflicts should be ignored or not, by default, it is set to `false` which means conflicts are not ignored.

this was not implemented yet in PyES, or I didn't find it?