[TEST] Added integration test for accessing highlighted fields when eager loading models

Closes #564
[TEST] Added integration test for sorting by multiple fields

Closes #596
Mention ActiveRecord specific hooks in the README.markdown

Mention that for ActiveRecord one should be using after_commit and after_rollback.

Closes #603.
Fixed typo in README ("powerfull" => "powerful")

Closes #614
[#571] Added support for replication=async option when storing document

Closes #571
[#572] Improving the percolation support in Tire::Model::Search and Tire::Model::Persistence

* Added integration tests for search and persistence
* Removed class variable which was cross-pollinating the class object space

Related #572
[#572] Do not use `@attributes` in Tire::Model::Search percolator integration

The `@attributes` Hash in Tire::Model::Search clashes with ActiveRecord namespace.

The Hash has been renamed to `@tire__attributes`

Closes #572
Return immediately from Index#bulk when passed collection is empty

Closes #576
[#593] Updated getting `MyModel.document_type` in Tire::Model::Naming

ActiveModel 4 changed the object returned from `Model.model_name`,
so let's call `to_s` on it explicitely.

Related: #593
[#593] Updated calling model callbacks in `MyModel.update_index`

Use the `run_callbacks` method, not `_run_update_elasticsearch_index_callbacks`.

See commit: rails/rails@19357a7b023fd56a5b381cd8894bb520c60cdb59

Authored by @hellvinz and @romansklenar.

Closes #593, closes #585
Use correct invocation of `include_root_in_json` in the `to_indexed_json` example

The previous example attempted to disable JSON roots but
didn't reference "self" so it only assigned a variable.

Closes #581
[TEST] Added a test for pagination in model search with :load
Merge branch 'master' into has_child