Multicurrency support

Attached is a patch that provides multicurrency support. If you've enabled Commerce Multicurrency and configured conversion rates, this patch will use those rates to convert between points and currencies. You still set one conversion rate between points and the default currency, but then this takes care of the rest.

It also includes a default rule that removes Userpoints line items from an order when the user switches currency, since the line item doesn't get converted during the currency switch and messes things up.

This is my first contributed patch so I probably didn't do this quite right but hopefully this is useful.