CRM-2935 Problem when using Pay Standard and cancelling the transaction half-way through Paypal

Check http://forum.civicrm.org/index.php/topic,2938.0.html

The fix is that we will only block repeated attempts to register for an event IF the pre-existing Registration Record has a status of "Registered" or "Attended". We will not block attempts to register if the existing registration record has status of Pending or Cancelled.

NOTE: For the case described in the forum post above... when a user Cancels the transaction before submitting it at the PayPal site - we do NOT get notification of the cancellation. Hence, the registration is left in a Pending status rather than a Cancelled status. If the same person attempts to re-register, they will be allowed to do so, and if they complete the transaction there will be 2 registration records for them - 1 Pending and 1 Registered.