Don't allow invalid coins

[alxwrd]

From the README:

When using the Coin factory class, a valid coin value should be used. Not doing so can create undesirable Coin objects.

 >>> coin = Coin(23)
OnePence(23)

# Use a CoinCollection instead!

>>> CoinCollection.from_value(23)
CoinCollection(TwentyPence(1), TwoPence(1), OnePence(1))

Re-thinking this I think it should be invalid to enter a state where an undesirable coin is about to be created (OnePence(23)).

I feel the correct way to handle this is to either raise an InvalidCoin exception. If a Coin has an .amount greater than 1, it's invalid.