Display the "you save" price as a currency amount

Currently the "you save" price is displayed as an unformatted amount - see http://imgur.com/oymJn

This change displays them as a currency formatted amount.

The second commit also fixes an issue with "You save" prices for variations where the prices are over 1000 - see http://getshopped.org/forums/topic/you-save-function-is-broken-for-prices-above-1000/#post-298823