Extension:OpenBadges/openbadges assertion table

Schema
mysql> describe openbadges_assertion; ++--+--+-+-++ ++--+--+-+-++ ++--+--+-+-++ 6 rows in set (0.01 sec)
 * Field             | Type             | Null | Key | Default | Extra          |
 * obl_id            | int(11)          | NO   | PRI | NULL    | auto_increment |
 * obl_timestamp     | binary(14)       | NO   | MUL | NULL    |                |
 * obl_receiver      | int(10) unsigned | NO   | MUL | NULL    |                |
 * obl_badge_id      | int(11)          | NO   |     | NULL    |                |
 * obl_badge_evidence | varchar(255)    | YES  |     | NULL    |                |
 * obl_expiration    | binary(14)       | YES  |     | NULL    |                |