Additional testnet fixes with HDWallet

These fixes cause the following tests to pass:

```javascript
var w = Bitcoin.Wallet("foobar", {network: "Bitcoin"})
assert(w.getMasterKey().priv.version == Bitcoin.Address.address_types['prod'])
w.generateAddress()
assert(w.getPrivateKey(0).priv.version == Bitcoin.Address.address_types['prod'])

var w = Bitcoin.Wallet("foobar", {network: "BitcoinTest"})
assert(w.getMasterKey().priv.version == Bitcoin.Address.address_types['testnet'])
w.generateAddress()
assert(w.getPrivateKey(0).priv.version == Bitcoin.Address.address_types['testnet'])
```

I would think this would be the behavior we'd want, correct?

(To be honest, I'm not sure if the change in fromBytes to specify testnet is necessary).