Add Serializer#map_properties to DRY property definitions

Adds the `#map_property` and `#map_properties` methods to `Oat::Serializer`.

These methods are to help DRY up easily mapped properties from an item to its serializer.

```ruby

class UserSerializer < Oat::Serializer
  include Oat::Adapters::HAL
  
  schema do
    # The long way
    property :id, item.id
    property :name, item.name
    property :email, item.email
  
    # the new short-hand
    map_properties :id, :name, :email
  end
end
```

Closes #5.