Add getPaymentDescription function to payment pclass

I just noticed that a useful helper function for payment processors is not in core - it's just a function for getting a useful description to send to processors.

I'll probably backport to 4.6 in the future after it has gone LTS in the interests of keeping developer consistency - or can do straight to 4.6. I'll duplicate it in my processors for now

/**

• Get description of payment to pass to processor.
  *
• This is often what people see in the interface so we want to get
• as much unique information in as possible within the field length (& presumably the early part of the field)
  *
• People seeing these can be assumed to be advanced users so quantity of information probably trumps
• having field names to clarify
  *
• @param array $params
• @param int $length
  *
• @return string
  */
  protected function getPaymentDescription($params, $length = 24) {
  $parts = array('contactID', 'contributionID', 'description', 'billing_first_name', 'billing_last_name');
  $validParts = array();
  if (isset($params['description'])) { $uninformativeStrings = array(ts('Online Event Registration: '), ts('Online Contribution: ')); $params['description'] = str_replace($uninformativeStrings, '', $params['description']); }

  foreach ($parts as $part)

  Unknown macro: { if ((!empty($params[$part]))) { $validParts[] = $params[$part]; } }

  return substr(implode('-', $validParts), 0, $length);
  }