Allow user to control contribution Receive Date for offline one-step Membership and Event Registration

As noted in this forum post - http://forum.civicrm.org/index.php/topic,2711.0.html - users often record membership+payment or event registration+payment on sometime after a payment is received. Currently, we force the contribution Receive Date to today's date when a Memership or Event Reg is created with "Record Payment" option.

Add "Received" field to the "Membership Payment and Receipt" section of the Membership form and to the "Payment Information" section of the Event Reg form - after the "Amount" field. Default value s/b today's date.