allow data bag item attributes encryption (only v1)

Allows creation of encrypted data bag items; at the moment only v1 is supported.

usage:

```ruby
data_bag = Ridley.new(config).data_bag.find(data_bag_name)
item = data_bag.item.new(id:  'my_id', password: 'sekret')
item.encrypt_attributes
item.save
```

Thanks,
Maciek