k should be 32 bytes, not 8 bytes

This is a bug with security implications. It is much easier to guess the value
of k within a 64 bit range. This would lead to compromised private keys.

The cryptography interface of bitcore is extremely poor. I recommend:
* Get rid of the C++ code, since it makes everything more difficult with little benefit
* Refactor all crypto, and have easily auditable bignum, point, ecdsa, and key classes
* Then actually audit the crypto