Add support for _timestamp field to @ElasticsearchMapping annotation.

Example usage:

    @ElasticsearchMapping(timestamp = true,
                          timestampFormat = "YYYY-MM-dd",
                          timestampPath = "publication_date",
                          properties = {...})

By default the "_timestamp" field of a mapping is disabled in Elasticsearch, and it's currently not possible to enable it using the `@ElasticsearchMapping` annotation. I've added a few fields to the annotation to make it possible to enable and configure it.

Relevant ES documentation page: http://www.elasticsearch.org/guide/reference/mapping/timestamp-field/