Dynamic default style

Dynamic configuration is enabled for different options, but :default_style is missing yet.
I'd like to use dynamic configuration for :default_style like the way below:

```ruby
class User < ActiveRecord::Base
  has_attached_file :avatar, :styles => lambda { |attachment| { :thumb => (attachment.instance.boss? ? "300x300>" : "100x100>") },
                             :default_style => lambda { |attachment| { attachment.instance.boss? ? :thumb : :large }
end
```

What might be missing yet in this pull request, is a change in documentation.