Bugfix and tests for bip32

Hi!

I've fixed what I think whas a bug in the implementation...
I was trying to run the tests from the spec, they didn't run and seems a modulo operation was missing.

I added the unit test for bip32 with the test vectors... I didn't check them with mocha though, I tested it on a external webpage. They should be good for you though.

Anyways, great job on the bip32 support, it rocks! Thanks a lot.

Cheers