Throw exception if we are changing initialized Jelly_Meta

[loonies]

Instead of silently ignoring propery change, Jelly_Meta::set should throw an exception if it is already initialized.

protected function set($key, $value)
{
    if ($this->_initialized)
        throw new Kohana_Exception('Jelly_Meta already initialized... bla, bla...');

    $this->{'_'.$key} = $value;

    return $this;
}