Ability to query rates for all service types available

**NOTE:** it now returns an Array of rates

## Getting shipping rates for all service types available:

    rates = fedex.rate(
      :shipper=>shipper,
      :recipient => recipient,
      :packages => packages,
      :shipping_details => shipping_details)

    rates.each do |rate|
      puts "#{rate.service_type} - #{rate.total_net_charge}"
    end

## Getting shipping rates for a single service:

    rates = fedex.rate(
      :shipper=>shipper,
      :recipient => recipient,
      :packages => packages,
      :service_type => "FEDEX_GROUND",
      :shipping_details => shipping_details)

    puts rates.first.rate.total_net_charge