support BIP32 absolute path in BIP32::build_key
updated exception message and added test for exception
BIP32 optimization
if this isn't the last derivation then the fingerprint is irrelevant so we can just spoof it!
that way we don't need the public key for the fingerprint
added more tests for BIP32::CKD
Merge pull request #35 from blocktrail/bip32-ckd-optimization

Bip32 ckd optimization