* Foreign Key XML support only needs to be added to the MDB2 XML Parser and Writer.
<code type="xml">
<index>
  <name>auth_user_id</name>
  <field>auth_user_id</field>
  <field>..</field>
  <foreign>
    <on>
      <table>foo</table>
      <field>bar</field>
      <field>..</field>
    </on>
    <update>cascade|restrict|null|default|noaction</update>
    <delete>cascade|restrict|null|default|noaction</delete>
  </foreign>
</index>
</code>
* Add the necessary MDB2 API calls in MDB2_Schema.
* Add Foreign Key support to MDB2 methods:
 * Manager Module Class
  * createConstraint()
  * deleteConstraint()
  * listConstraints()
 * Reverse Module Class
  * getTableConstraintDefinition()

References:
http://dev.mysql.com/doc/mysql/en/innodb-foreign-key-constraints.html
http://www.postgresql.org/docs/8.0/interactive/ddl-constraints.html#DDL-CONSTRAINTS-FK