JS: Add is_valid() for UInt160 and Currency.
JS: Add is_valid() for UInt160 and Currency.

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