title: BrainTree: get all customer's payment methods
body: <p>There is a way to get a list of all cards:</p>
  
  <pre><code>gateway.creditCard.expiringBetween(year1900, year2100, function (err, result) {...})
  </code></pre>
  
  <p>and then call <code>paymentMethod.find</code> for each individual card.  But I would like to get all payment methods associated with a customer, in one call, is this possible?</p>