Added support for addOverride method to allow for association and attribute overrides.

Issue #29 requested the addition of a method that would allow one to remove an association so that it could possibly be re-added. That issue was closed out of fear of possibly breaking code.

I propose an alternative, which I feel is the better way to handle this, and that's to add a method called addOverride to both DoctrineCollector and DoctrineORMMapper, which will allow one to override both attributes and associations as necessary in a Doctrine supported manner.

An example in the code that I'm using is as follows:

    $collector = DoctrineCollector::getInstance();

    $collector->addOverride($config['class']['user'], 'setAssociationOverride', array(
        'fieldName'   => 'groups',
        'joinColumns' => array(
            array(
                'name' => 'user_id',
                'referencedColumnName' => 'USER_ID',
                'onDelete' => 'CASCADE'
            )
        )
    ));

This allows one to call setAssociationOverride and setAttributeOverride as necessary to achieve what ever overrides are needed in your application.