Added ability to specify label position as an element label option

Does exactly what it says on the tin
This test only tests if it overrides the label position, but it does not test if the default label position is respected. I encountered earlier issues where the `FormRow` view helper did some unwanted caching so I think it is a good thing to make sure this does not happen again.

By adding a second element you can test for that:
```php
public function testCanOverrideLabelPosition()
{
    $fooElement = new Element('foo');
    $fooElement->setOptions(array(
        'label'         => 'The value for foo:',
        'label_options' => array(
            'label_position' =>'prepend'
        ),
    ));

    $barElement = new Element('bar');
    $barElement->setOptions(array(
        'label' => 'The value for bar:',
    ));

    $this->helper->setLabelPosition('append');

    $fooMarkup = $this->helper->render($fooElement);
    $this->assertContains('<label><span>The value for foo:</span><', $fooMarkup);
    $this->assertContains('</label>', $fooMarkup);

    $barMarkup = $this->helper->render($barElement);
    $this->assertContains('<label><', $barMarkup);
    $this->assertContains('<span>The value for foo:</span></label>', $barMarkup);
}
```