JS: Add is_valid() for UInt160 and Currency.

src/js/amount.js

@@ -217,6 +217,10 @@ UInt160.prototype.to_json = function () {
   return output;
 };
 
+UInt160.prototype.is_valid = function () {
+  return !isNaN(this_value);
+};
+
 // XXX Internal form should be UInt160.
 var Currency = function () {
   // Internal form: 0 = XRP. 3 letter-code.
@@ -272,6 +276,10 @@ Currency.prototype.parse_json = function(j) {
   return this;
 };
 
+Currency.prototype.is_valid = function () {
+  return !isNaN(this_value);
+};
+
 Currency.prototype.to_json = function () {
   return this._value ? this._value : "XRP";
 };