Add backburner support for async reindex

Add backburner support for async reindex

in your init file, e.g. `config/initializer/searchkick.rb`

```ruby
require "searchkick/backburner"

Searchkick.backend = Searchkick::Backburner::Backend.new
Searchkick.reindex_job = Searchkick::Backburner::ReindexJob
```

@ankane 
Need your feedback for gem dependency, code organization, and test code.