BIP32 speed improvement

* remove multiple concats with one, more efficient, concat
* allow generating blank BIP32 for internal use
* other minor improvements

Speed test:
```
var d1 = new Date();
for (var i = 0; i <= 10000; i++) {
  var bip32 = new bitcore.BIP32();
  var bip32b = bip32.derive('m/5');
  console.log(i);
}
var d2 = new Date();
console.log(d2-d1);
```

This code took 32 seconds on master, and 20 seconds on this new branch on my machine (in node). So things are about 30% faster.