Add Must* versions of methods that return error

[jpap]

Thanks for a great package!

It would be quite convenient to have methods on Money that panic instead of returning error. These could wrap the existing methods, with the Must prefix. For example, MustAdd would wrap Money.Add.

In my program, for example, I deal with just one currency (and am using go-money for correct handling of cents in the dollar). The ability to avoid explicit error checking, and use the result of Add, Subtract, etc. as an unary expression would be very helpful.