bugfix: allow for testnet BIP32 address generation

[nmarley]

This passes the @network of Keychain on thru to BTC::Key. Currently
testnet BIP32 keychains will generate mainnet addresses.

[nmarley]

Currently:

$ irb -rbtcruby
>> kc = BTC::Keychain.new(xpub: 'tpubD6NzVbkrYhZ4XgiXtGrdW5XDAPFCL9h7we1vwNCpn8tGbBcgfVYjXyhWo4E1xkh56hjod1RhGjxbaTLV3X4FyWuejifB9jusQ46QzG87VKp');nil
=> nil
>> kc.derived_key('0/0').address.to_s
=> "12CL4K2eVqj7hQTix7dM7CVHCkpP17Pry3"

After patch:

irb -I./lib -rbtcruby
>> kc = BTC::Keychain.new(xpub: 'tpubD6NzVbkrYhZ4XgiXtGrdW5XDAPFCL9h7we1vwNCpn8tGbBcgfVYjXyhWo4E1xkh56hjod1RhGjxbaTLV3X4FyWuejifB9jusQ46QzG87VKp');nil
=> nil
>> kc.derived_key('0/0').address.to_s
=> "mgiHMN7dJsANUWwLfgbiw7hc4kR5xMjPhw"
>>

[oleganza]

Thanks!