Use Node.js crypto ripemd160 hash if available.

lib/crypto/hash.js

@@ -27,11 +27,20 @@ Hash.sha256sha256 = function(buf) {
   return Hash.sha256(Hash.sha256(buf));
 };
 
+Hash.ripemd160 = function(buf) {
+  $.checkArgument(BufferUtil.isBuffer(buf));
+  return crypto.createHash('ripemd160').update(buf).digest();
+};
+
+// Node.js crypto ripemd160 hashes are not supported in a browser
+// We'll replace with a (slower) version that does.
+if (global.window) {
   Hash.ripemd160 = function(buf) {
     $.checkArgument(BufferUtil.isBuffer(buf));
     var hash = (new hashjs.ripemd160()).update(buf).digest();
     return new Buffer(hash);
   };
+}
 
 Hash.sha256ripemd160 = function(buf) {
   $.checkArgument(BufferUtil.isBuffer(buf));