add toString method for bip32

...so that it is consistent with the rest of the library

lib/bip32.js

@@ -308,6 +308,16 @@ BIP32.prototype.deriveChild = function(i) {
   return ret;
 }
 
+BIP32.prototype.toString = function() {
+  var isPrivate =
+    (this.version == constants.mainnet.bip32privkey ||
+      this.version == constants.testnet.bip32privkey);
+
+  if (isPrivate)
+    return this.extendedPrivateKeyString();
+  else
+    return this.extendedPublicKeyString();
+};
 
 function uint(f, size) {
   if (f.length < size)

test/test.bip32.js

@@ -1,4 +1,5 @@
 var should = require('chai').should();
+var constants = require('../lib/constants');
 var BIP32 = require('../lib/bip32');
 
 describe('BIP32', function() {
@@ -299,4 +300,31 @@ describe('BIP32', function() {
     });
   });
 
+  describe('#toString', function() {
+    var bip32 = new BIP32();
+    bip32.fromRandom('mainnet');
+    var tip32 = new BIP32();
+    tip32.fromRandom('testnet');
+
+    it('should return an xprv string', function() {
+      bip32.toString().slice(0, 4).should.equal('xprv');
+    });
+
+    it('should return an xpub string', function() {
+      var bip32b = new BIP32(bip32.extendedPublicKeyString());
+      bip32b.toString().slice(0, 4).should.equal('xpub');
+    });
+
+    it('should return a tprv string', function() {
+      tip32.toString().slice(0, 4).should.equal('tprv');
+    });
+
+    it('should return a tpub string', function() {
+      var tip32b = new BIP32(tip32.extendedPublicKeyString());
+      tip32b.toString().slice(0, 4).should.equal('tpub');
+    });
+
+
+  });
+
 });