[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
[DOC] Added caveats and info about the import Rake task and `find_in_batches`

Related: #530, #649
On Ruby 1.8, Only require URI methods from Rack if they are not already loaded

Closes #619
Added Ruby 2.0 to Travis test matrix
[TEST] Allow failures on REE with TravisCI
Added support for "Put Mapping" API

See: http://www.elasticsearch.org/guide/reference/api/admin-indices-put-mapping.html

Related: #645, #490, #194

Closes #520
Refactored the "Put Mapping" API support

* Cleaned up and simplified code
* Made the return values more compatible with the rest of the API (return `false` for failures, users should check `index.response`)
* Removed `put_mapping!`
* Amended, completed and polished tests
* Updated documentation

Related: #194, #490, #520, #645
Added support for "Delete mapping" API

Fixes: #520
Refactored the "Delete mapping" API support

Related: #520