Added FK functions in DBUtil

[axelitus]

Usage:

ADD FKs CONSTRAINTS

\DBUtil::add_fk('my_table', array(
    'fk_1' => array(
        'field' => 'my_field',
        'references' => array(
            'table' => 'referenced_table',
            'field' => 'referenced_field'
        ),
        'on_delete' => 'cascade',
        'on_update' => 'cascade'
    ),
    'fk_2' => array(
        'field' => 'my_field',
        'references' => array(
            'table' => 'referenced_table',
            'field' => 'referenced_field'
        ),
        'on_delete' => 'cascade',
        'on_update' => 'cascade'
    ),
));

DROP FKs CONSTRAINTS

\DBUtil::drop_fk('my_table', array('fk_1', 'fk_2'));

[dhrrgn]

This will not be merged until doc blocks are added. Also, please document the functions and submit a pull request on github.com/fuel/docs

[axelitus]

Doc blocks were added and a pull request in fuel/docs has been requested (fuel/docs#106)

[calvinfroedge]

I did not look at pull requests before I wrote functionality for this also. I guess I should have = (

Maybe we discuss which approach to take? Here is my pull request:

#245

I can close mine if needed.

[axelitus]

I will be removing this repo (messed things up, still learning git...) but I'm guessing this functionality will be present in the new DBUtil drivers that are planned for release 1.1

I have made a backup of this code though if needed...