verifymessage shouldn't check priv keys

Updated `VerifyMessage` to avoid checking private key and just compare the signature pubkey with the given address.

Refs #123 
addr was already type asserted as an AddressPubKeyHash.  A better way to do this would be to declare a variable in the switch statement:

```Go
switch checkAddr := addr.(type) {
case *btcutil.AddressPubKeyHash:
    // Use checkAddr as a *btcutil.AddressPubKeyHash
case *btcutil.AddressPubKey:
    // Use checkAddr as a *btcutil.AddressPubKey
}
```