Renaming Path to Bip32Path, invalid to fixtures

Only thing I haven't been able to do is to add "undefined" to fixtures. So I could not properly test
that node.derivePath() properly fails as it should. However, I added "null" there, and "null" and "undefined"
behave in similar way in JavaScript, so that should catch that.

src/hdnode.js

@@ -292,7 +292,7 @@ HDNode.prototype.isNeutered = function () {
 }
 
 HDNode.prototype.derivePath = function (path) {
-  typeforce(types.Path, path)
+  typeforce(types.Bip32Path, path)
 
   var splitPath = path.split('/')
   if (splitPath[0] === 'm') {