Improve the `tire:import` Rake task

I wanted to submit this as a topic for conversation. This solution provides the following:

* Detection of models using Tire::Model::Search.mapping, auto-indexes all of these by default.
* You can still pass CLASS var, but it now accepts multiples.
* Removed Kaminari, not sure why it was needed in this task.
* Uses the progress_bar gem, a very clean implementation IMHO.