Include 0x in the parent fingerprint
Add PrivateKey.__sub__
Add crack_private_key
Python3 compatibility
Update README to reflect dangers of revealing non-prime private children
More py3 compat
Add link to article about cracking the private key
Merge pull request #23 from sbuss/crack_private_key

Crack private key

Thanks to http://bitcoinmagazine.com/8396/deterministic-wallets-advantages-flaw/ for the information.