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
yeah, you're right too
it's more simple

actually, why i'm using `if elsif` because i worried to another database sql syntax
maybe like this

    if :pg      
    elsif :mysql2
    elsif :sqlite3
    ....
    end

but because currently known issues only mysql and postgresql, so i think your solution is better suited