Bugfixes and tests for bip32

Fixed a few errors on bip32 support:
 * missing modulo operation on ckd private derivation [9fd46c2]
 * fixed bad check for private derivation on public key [14803d0]
 * fix deserialization from master public key [26e5914]

Also added an unit test for bip32 with the test vectors [740a0fc, f9cbcbe]... I didn't check them with mocha though, I tested it on a external webpage. They should be good or need small changes.

Anyways, great job on the bip32 support, it rocks! Thanks a lot.

Cheers