Add endpoint to get transmission details

There is currently no way to get transmission details for a fax.  Adding the following will get the status.  Then you can poll the transmission status if you want to send a fax synchronously.

As a quick example:

    transmission_guid = hello_fax.send_fax(number, pdf)['Tramisssion']['Guid']
    while true
      transmission_status = hello_fax.transmission_details(transmission_guid)
      return transmission_status['Transmission']['StatusCode'] unless ['T','P'].include?(transmission_status['Transmission']['StatusCode'])
    end