Tests: Added bitcoind base58 key json data and tests

It seemed weird to me too at first, but let's think of UX first.

This looks clear to me:
```
var pk = new PrivateKey('random');
```
but this is not clear to produce a random private key:
```
var pk = new PrivateKey(PrivateKey.Random);
```

The `_.isUndefined(data)` correction seems fair though