JS: Add is_native() to Amount.
JS: Add is_native() to Amount.

ripple/ripple-lib,darkdarkdragon/ripple-lib,wilsonianb/ripple-lib,darkdarkdragon/ripple-lib,wilsonianb/ripple-lib,wilsonianb/ripple-lib,ripple/ripple-lib,ripple/ripple-lib,darkdarkdragon/ripple-lib,ripple/ripple-lib