Async Reindex :: Added support for Resque

Hi Ankane

Resque is also a preferred background processor, I thought It would be great to have resque support with Searchkick so have added support for Resque for Asynch ReIndexing. Also introduce one installer to create a initializer(searchkick.rb) where user can opt for his backgrounder (Delayed/Resque). I kept Delayed Job as default backgrounder. Initializer also have option to configure the Elastic search url. By default it takes it from ENV.

Please review the changes and let me know if I missed anything.