Allow event to have multiple registrations from same email address and allow additional participants to be added without email addresses

When using the "Register Additional Participants" feature - provide the option to either allow the same email address for additional participants OR not require an email address. Because we need either an email address or first/last name to create a contact - email address will be optional IF a profile with first and last name fields is included in the event registration configuration.

This has come up recently (http://forum.civicrm.org/index.php/topic,6725.html), and adding support to it should not be too hard.

1. add a new boolean column to civicrm_event called allow_same_participant_emails whose default is 0. Ensure you also add it to the incremental upgrade file

2. In the event creation workflow add this checkbox (default unchecked)

3. In the event registration code: if this column is set, then disable the check for registration in CRM/Event/Form/Registration/Register.php, function checkRegistration

4. in the event confirmation code: CRM/Event/Form/Registration.php, line 672, do not retrieve prior participant id (that code does not seem to belong there anyway. We should consider deleting that code (http://forum.civicrm.org/index.php/topic,6211.msg28791.html#msg28791). On more reflection this does seem like a bug and that code should be deleted.

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We should make sure that this change supports the use case(s) described in this forum post:
http://forum.civicrm.org/index.php/topic,4184.0.html

... and check the code patches posted to the wiki by 'tabidi' which seem to have been picked up by several different people - so that hopefully these changes bring support for that workflow into core:
http://wiki.civicrm.org/confluence/display/CRMDOC/Multi+User+Registration+with+1+Email