[feature] Adds metafields - pre release

* Adds CustomerMetafield and SubscriptionMetafield resources
* Updates to activeresource 3.2.16+ for bugfixes and to allow metafield
  resources.

This is a pre-release that adds customer and subscription metafield
endpoints.

Example usage:

(all examples work with SubscriptionMetafield or CustomerMetafield I
chose to show CustomerMetafield for brevity)

Creating a field

```ruby
field = CustomerMetafield.build(name: 'shoe size')
field.on_hosted_pages = product_id_1, product_id_2
field.on_csv_export   = false
```

Note that build is being used here instead of create. the `on_*` methods
are helper methods. Without them it is painful to modify the scope of
the metafield.

Updating a field

```ruby
metafields = CustomerMetafield.all
field      = metafields.first

field.name            = "new name"
field.on_hosted_pages = product_id_1, product_id_2, ...
field.on_csv_export   = true

field.save
```

Deleting a field

```ruby
metafields = CustomerMetafield.all
field = field.first
field.destroy
```

Note: finding an indivdual metafield is not supported. You will have to
either create a record or use `all` and then find the record that way.

It also might surprise you if you use `create` passing the `current_name`
attribute for an existing field it will update the record instead of
create a new one. Please see the API documentation for further information.