Initial example for configurable product

This is the inital example of a configurable product fixture. The format I have gone with it is:
```yaml
  configurable_attributes:
      <sku>:
        <attribute_code>: <label>
```
The default configurable product YAML file looks like this:

```yaml
catalog/product/configurable:
  default_attributes:
    sku: test-conf
    attribute_set_id: 9
    name: product name
    weight: 2
    price: 100
    description: Product description
    short_description: Product short description
    tax_class_id: 4
    type_id: configurable
    visibility: 4
    status: 1
    stock_data: { use_config_manage_stock: 0, manage_stock: 1, is_in_stock: 1 }
    website_ids: [1]
  simple_product_attributes:
    website_ids: [1]
    attribute_set_id: 9
    name: product name
    tax_class_id: 4
    status: 1
    weight: 2
    price: 100
    description: Product description
    short_description: Product short description
    type_id: simple
    visibility: 1
    stock_data: { use_config_manage_stock: 0, manage_stock: 1, min_sale_qty: 1, max_sale_qty: 2, is_in_stock: 1, qty: 999 }
  configurable_attributes:
       test-conf-1:
        color: Green
      test-conf-2:
        color: Blue
      test-conf-3:
        color: Black
```

A main 'gotcha' I came across was making sure the configurable attribute you are defining in your fixture, is added to the attribute set you are using. 

The example default YAML file uses 'color' which is defined in the 'default' attribute set. The attribute set id can be noted down from the URL in the admin, when visiting the attribute set.

Need to work on clearing up associated simple products