Added support for reading private keys as well
Fixed key fingerprinting code

RFC 480 states that "a V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99, followed by the two-octet packet length, followed by the entire Public-Key packet starting with the version field." This apparently means that, in the case of a private key packet, all additional content should be ignored.
Other implementations seem to interpret it this way too...