Persistence implementation: repository pattern

The `persistence` branch contains the "elasticsearch-persistence" Rubygem, which provides a number of features and facilities to store and retrieve Ruby domain objects in Elasticsearch, via the [repository](https://www.google.com/search?q=repository+pattern) pattern.

Please see the [README](https://github.com/elasticsearch/elasticsearch-rails/blob/persistence/elasticsearch-persistence/README.md#usage), the code annotations and the unit tests for detailed information.
The repository implementation will be used to provide an ActiveRecord-like integration with Ruby domain objects,
similar to the old `Tire::Model::Persistence`.

## TODO

- [ ] Example application
- [ ] Integration tests
- [ ] ActiveRecord pattern