object syntax to allow setting current object

This change allows you to set `@current_object` with the `object` method in order to use the `attributes` method in a top-level keyspace.

```ruby
Person = Struct.new(:name, :age, :employer)
person = Person.new('alex', 28, 'Zaarly')

object person do
  attributes :id, :name, :employer
end
```

This will output the following json document:

```json
{
  "name": "alex",
  "age": 28,
  "employer": "Zaarly"
}
```