CRUD helper insert / update: support timestamp_column = NOW()

[korpa]

CRUD helpers should support mysql's "NOW()"

[nramenta]

Added in version 0.5.0. You can now use literal helpers like so:

$db->insert('posts', array(
    'title'      => 'Hello, World',
    'created_at' => $db->literal('NOW()')
));

[korpa]

Thank you very much