Auto-generation of UUID keys

```ruby
class User < Cequel::Model::Base
  key :id, :uuid, :auto => true
  column :name, :text
end
```

Should auto-populate the UUID when the instance is created (as a new record).

Probably the best way to do this is to allow the `:default` option to be a `Proc`, which the `:auto => true` would then just set up for you.