Fixes #167

This pull request fixes the multiplication order in `HDWallet.derive` for extended public keys, which until now, has clearly not been under test.
The `ECPointFp` (the type of a typical public key)  class only supports multiplication of the form `ECPointFp.prototype.multiply :: ECPointFp -> BigInteger -> ECPointFp`.
Therefore, as pointed in #167, when done in the commutative form, the `BigIngeter.prototype.multiply` fails. (`ECPointFp` missing `BigInteger` methods etc).

Better tests will follow up eventually to address #169.