Fixes #167

This pull request fixes the multiplication order in `HDWallet.derive` 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 :: BigInteger -> ECPointFp`.
Therefore, as pointed in #167, when done in the commutative form, the `BigIngeter.prototype.multiply` fails (missing methods).