Support Testnet Extended Keys
The following HD version bytes are unimplemented:

Coin | Public Key | Private Key | Address Encoding | BIP 32 Path
-- | -- | -- | -- | --
Bitcoin Testnet | `0x043587cf` - `tpub` | `0x04358394` - `tprv` | P2PKH or P2SH | m/44'/1'
Bitcoin Testnet | `0x044a5262` - `upub` | `0x044a4e28` - `uprv` | P2WPKH in P2SH | m/49'/1'

This results in the following errors:
1. Inability to generate any testnet extended public or private keys using `new()` or `from_str` methods
2. Attempting to derive either of the above BIP 32 Path's will result in an incorrect Mainnet key generation.