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
As this exactly the same method call, and the only varying piece is the `as('something')` part. I think we can simplify it like:

```ruby
  type_cast = @config.db_adapter == :pg ? 'VARCHAR' : 'CHAR'
  casted_column = ::Arel::Nodes::NamedFunction.new('CAST', [model.arel_table[column.to_sym].as(type_cast)])
```

what do you think?