Need to work with currency

[taylodl]

Basic implementation works, but currency handling isn't quite where it should be

The monthly payment, monthly principle, and monthly interest should all be in currency = not floats. The numbers should add as expected. The client won't like numbers that don't add, not even for a POC.

[taylodl]

That was a PITA, but it's done. Haven't really had to worry about currency before. Gotta get the numbers right and make sure everything adds up!

Also fixed a minor mistake in making the monthly payment calculation. Weird - for some loans it was right, for others it was wrong. I forced the order of precedence to make sure the calculation was right and that fixed things.