Generalize to other cryptocurrencies

[clarkmoody]

The WitnessProgram::from_address function is the only place where constraints on the human-readable part are enforced.

Feature wish-list:

• [x] Maintain list of cryptocurrencies that defines hrp for mainnet and testnet
• [x] enum for networks, Function return String with hrp value for Network
• [x] Function to classify a hrp
• [x] WitnessProgram gains additional network field. to_address loses hrp parameter, which is automatically set by value of network