Handle comma-delimited amounts with Valitor

The Valitor gateway notification code always expects an amount specified in cents. This looks to be true for most of the time, however this amount can also be sent with a comma delimiter.

Here are some recent values from Valitor for the 'Upphaed' field (used by the Notification#gross method).

![](http://f.cl.ly/items/263n2S1L1N3H3V0L0e2x/Screen%20Shot%202013-12-11%20at%2012.29.18%20PM.png)

This PR will convert these comma-delimited values to the generally expected decimal-delimited format that Shopify et al expects.

Please review: @Soleone @melari