[48480747] fix crash when deleting job opening with interviewers assigned

We need an 'id' column for join_table so that children can be auto-deleted when deleting parent.
So add an id column for table opening_participant.

As this change is sort of destructive, so merge scheme definition.

Signed-off-by: Henry Chen <hchen@vmware.com>
Merge branch 'master' of https://github.com/vmw-tmpst/heroapp-LetsHire into id_column

Conflicts:
	db/migrate/20130411063008_add_columns_to_opening_and_add_assessment.rb

Signed-off-by: Henry Chen <hchen@vmware.com>
sync with master

Signed-off-by: Henry Chen <hchen@vmware.com>
Merge pull request #113 from Henry-CF/id_column

[48480747] fix crash when deleting opening with interviewers assigned