Implementing address_byte_prefix for addresses/transactions

[Lvl4Sword]

• address_byte_prefix is now available as an option for transactions and addresses through setting the address_byte_prefix parameter. If address_byte_prefix isn't set, just use MAINNET_ADDRESS_BYTE_PREFIX.
• Blanket imports are removed and replaced for what actually gets imported.
• Unnecessary () is removed from classes

This is an initial stab at #26 -- Doesn't handle segwit, testnet, or any custom script prefixes.