Added optional callback

Adds an optional callback so you can do custom stuff with the model before it's created.

```ruby
namespace :db do
  task :populate => :environment do
    populate User, 10..50
    populate Sport, 3..5
    populate Team, 20..50
    populate Player, 200..500 do |p|
      p.image_url = "http://upload.wikimedia.org/wikipedia/commons/d/d1/Brandon_Roy_Wizards.jpg"
    end
  end
end
```