Issues/78 configurable fixtures

re #78 adding fixture processing for configurable products. specify the relevant attributes by code and child products by id e.g.
    - entity_id: 410955
      attribute_set_id: 4
      type_id: simple
      sku: "Test Simple Child"
      name: "Test Simple Child"
      short_description: "Test Simple Child"
      description: "Test Simple Child"
      url_key: test-child
      website_ids:
        - us
      category_ids:
        - 2 # Default Category
      price: 30.00
      special_price: 30.00
      tax_class_id: 2 # Taxable Goods
      status: 1             # Enabled
      visibility: 4         # Visible in Catalog & Search
      color: 4         # Configurable Super Attribute
    - entity_id: '336835'
      attribute_set_id: 4
      type_id: configurable
      sku: "test-configurable"
      name: "Test Configurable"
      short_description: "Test Configurable"
      description: "Test Configurable"
      url_key: test-configurable
      super_attributes:
        - color
      configurable_children:
        - 410955
      website_ids:
        - us
      category_ids:
        - 2 # Default Category
      price: 30.00
      special_price: 30.00
      tax_class_id: 2 # Taxable Goods
      status: 1             # Enabled
      visibility: 4         # Visible in Catalog & Search