title: Generating unique tracking numbers
body: <p>I am building a small rails app and I need a way to generate tracking numbers to give to customers to check up on their status.  I want something similar to what the major shipping companies use, but not as long.  Im thinking 12-16 characters long is fine, but I dont want something like 0000000001 where people can just type in incremented numbers and peep around.  </p>
  
  <p>What is a good way to generate unique number/letter combination tracking codes?</p>