Allow customer references to have arbitrary type

Current `fedex` code sends any customer reference numbers to FedEx with the `CUSTOMER_REFERENCE` type. But FedEx supports more types, including `RMA_ASSOCIATION` which we happened to need.

This code allows any entry in the `customer_references` array to be a Hash with `:type` and `:value` entries, which will be passed through to FedEx Web Services as `CustomerReferenceType` and `Value`. The old syntax is still supported and has the same behavior -- the default type is `CUSTOMER_REFERENCE`.

Example of the new usage:

```ruby
shipping.label(
  format: 'pdf',
  filename: 'label.pdf',
  # ...
  packages: [
    {
      weight: {value: 1, units: 'LB'},
      customer_references: [
        "Order #12345", # default is CUSTOMER_REFERENCE
        {type: 'RMA_ASSOCIATION', value: 'RA_2468'} # custom type
      ]
    }
  ]
)
```

Also, this commit enables the correct spelling `customer_reference` while still supporting the old `customer_refrence`.

Thanks for the fedex gem; it is very helpful!