add preference in the list of search parameters

[alexis779]

Add the preference feature described here:
http://www.elastic.co/guide/en/elasticsearch/reference/1.4/search-request-preference.html

Note that 2 unit tests fail.

bundle exec rake test:unit

ERROR (0:00:02.419) test_: Collection with ActiveModel::Serializers should be serializable. 
      uninitialized constant ActiveModel::ArraySerializerSupport
    @ test/unit/results_collection_test.rb:402:in `block (3 levels) in <class:ResultsCollectionTest>'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:436:in `instance_exec'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:436:in `block in run_current_setup_blocks'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:434:in `each'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:434:in `run_current_setup_blocks'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:411:in `block in create_test_from_should_hash'
      /var/lib/gems/2.1.0/gems/mocha-0.14.0/lib/mocha/integration/mini_test/version_2112_to_320.rb:40:in `run'

ERROR (0:00:02.442) test_: Item with ActiveModel::Serializers should be serializable. 
      undefined method `attribute' for MyItemSerializer:Class
    @ test/unit/results_item_test.rb:195:in `<class:MyItemSerializer>'
      test/unit/results_item_test.rb:194:in `block (3 levels) in <class:ResultsItemTest>'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:436:in `instance_exec'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:436:in `block in run_current_setup_blocks'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:434:in `each'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:434:in `run_current_setup_blocks'
      /var/lib/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:411:in `block in create_test_from_should_hash'
      /var/lib/gems/2.1.0/gems/mocha-0.14.0/lib/mocha/integration/mini_test/version_2112_to_320.rb:40:in `run'

[karmi]

Hi, thanks for the patch, but the codebase is effectively frozen, especially when it comes to adding or expanding features...