add initializer generator

this is pull request for issue number https://github.com/antillas21/ajax-datatables-rails/issues/43 


the idea is create config file to set db adapter, so we can check the db adapter on AjaxDatatablesRails::Base class
Can we add a note here like:

```ruby
# available options for db_adapter are: :pg, :mysql2, :sqlite
config.db_adapter = :pg
```

Though it would be even cooler to make the generator read the `Gemfile` and search for 'mysql2', 'pg', or 'sqlite'... and depending on what it found, set the corresponding symbol here.

But not a requirement.