add support for arbitrary version bytes

[Kefkius]

Add two classes, CoinPrivateKey and CoinPublicKey, that take a version byte parameter in their constructors.
Also allow WIF version byte to be specified via a parameter.

[shea256]

Hi @Kefkius thanks for the suggestions and pull request. I have a few questions:

1. Do you find the method that you've included here to be better than this:

class KefkiusCoinPrivateKey(BitcoinPrivateKey):
    _pubkeyhash_version_byte = 42

kefkius_priv_1 = KefkiusCoinPrivateKey()
kefkius_priv_2 = KefkiusCoinPrivateKey()

Here, one can create a new class for a new coin and then create new instances of the class. This is in contrast to your pull request, where you allow users to create new keys without defining a new class.

1. In my experience, WIF version numbers are simply equal to 128 plus the pubkeyhash version number. Have you been in a situation where you'd need to explicitly set the WIF version number separate from the the pubkeyhash version number?

[Kefkius]

@rxl You're welcome. I see that the method you described is at least as useful as mine to a user. I made the pull request after seeing that so many coins were on "to-do" implementation status. I figured this would be an easy little hack to use, but yes, the method you described is just as effective, and may be a better option.

As for WIF versions: Yes, I've had to explicitly define a WIF version.

[CLAassistant]

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