Callback Improvement

Currently, Model::Callbacks updates the index on any change to the model even if the attributes being changed are not part of the index being updated. This PR includes changes that allow the user to specify if the Model should update ElasticSearch on indexed-attribute changes only.

In the below example, only changes to the name attribute will cause the ElasticSearch server to be updated.

```
class SomeModel < ActiveRecord
  should_tire_only_update_on_index_changes true
  tire.mapping do
    indexes :name, analyzer: 'snowball'
  end
end    
```