Added support for none active record model, which has no after_commit callabck.
This commit also includes the following changes -

  1. Refactored worker now
  2. Refactored async_callbacks now it's using ActiveSupport::Concern
  3. Added specs for after_destroy
Fixed issue with :delete operation
Merge branch 'active_model_support'
Removed unnecessary puts from update_index worker.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/tire_async_index/workers/update_index.rb
Removed Kernal.const_get because it works only on Ruby 2.x version. (ActiveSupport dependency is not an option)
Added recursively constant lookup option so that it can load const from multiple namespace.
Merge pull request #7 from we4tech/compatible_with_1_9

Compatible with 1.9.x