(FR #1771) Add "Bitcoin" payment instrument

Change-Id: Id6b99909b42f0b6b87d7d6105bb41002b3b8820a
(FR #1771) Only perform settlement conversion if != USD

We have been recalculating settlement conversion even when the gateway
already gave us the settlement numbers.  This is bad.

This patch is a small band-aid which will not reconvert USD.

Change-Id: I423c99c1a9b2c20af9a666b2b8087021e91fa01c