Added support for elasticsearch 1.0 or newer.

Provides an `ElasticsearchTarget` and a `CopyToIndex` template task.

Modeled after `luigi.contrib.rdbms.CopyToTable`.

A minimal example (assuming elasticsearch is running on localhost:9200):

    class ExampleIndex(CopyToIndex):
        index = 'example'

        def docs(self):
            return [{'_id': 1, 'title': 'An example document.'}]

    if __name__ == '__main__':
        task = ExampleIndex()
        luigi.build([task], local_scheduler=True)

Tests can be run against a local elasticsearch index.