Allow multiple people to contribute to one participants event registration

We have need to stop CiviCRM from checking for duplicates when registrants are signed up for an event. The use case is we have multiple people paying into a single registration, ie parents sign a kid up and a local church/foundation also kicks in $$ towards the participation.

I'm 99% sure we can handle the rest of the workflow issues, like appending payments, doing my own dupes check etc.. using hooks & apis, but the fact that the checkregistration() function is called by preprocess() CRM_Event_Form_Registration_Register is a show stopper.

I would like to add a public variable to the CRM_Event_Form_Registration_Register $_skipDupeRegistrationCheck and add if statements to the calls to self::checkregistration() function calls.

The attached patch will allow developers to stop the dupe check using hook_civicrm_buildForm()

Patch against the 3.4 trunk, ie revision 32632