Add support for FedEx freight economy and priority

Adds rate request support for FedEx Freight Economy and Priority provided that a freight_options hash is supplied to the find_rates method.

Example:

```ruby
freight_options = {
      account: '5555', # freight account number
      billing_location: billing_location, # billing location associated with the account number
      payment_type: 'SENDER',
      freight_class: 'CLASS_050',
      packaging: 'PALLET',
      role: 'SHIPPER'
}

response = @carrier.find_rates(origin, destination, packages, { :freight => freight_options })
```

Specifying freight options will __only__ return freight economy and priority rates.

Feedback is appreciated and thanks in advance! :)