[-] MO : Proper negation of voucher discounts

Besides being more elegant and correct, this way enables developers
to create custom behaviors with Cart Rules, where instead of offering
discounts, they could increase the price. However, this change will not
effect anybody else.

See for example these screenshots:
http://cl.ly/image/0e0g2D1u0Y45 (before)
http://cl.ly/image/3b020a303N2v (after)

You can see how now the normal case is still a normal minus, while with
our custom built override we have a positive value, that is they way
it's meant to be.

mailalerts.php

@@ -325,7 +325,7 @@ public function hookActionValidateOrder($params)
 			$items_table .=
 				'<tr style="background-color:#EBECEE;">
 						<td colspan="4" style="padding:0.6em 0.4em; text-align:right;">'.$this->l('Voucher code:').' '.$discount['name'].'</td>
-					<td style="padding:0.6em 0.4em; text-align:right;">-'.Tools::displayPrice($discount['value'], $currency, false).'</td>
+					<td style="padding:0.6em 0.4em; text-align:right;">'.Tools::displayPrice(0 - $discount['value'], $currency, false).'</td>
 			</tr>';
 		}
 		if ($delivery->id_state)