Added RFC 6979 calculation for K in ecdsa signatures

Hello, I've been using your excellent library for an independent validation of my HW wallet prototype. My HW  wallet uses the RFC6979 method for a deterministic K in ecdsa signatures. See the RFC for more detail. There is also a tweak to ensure that 's' is even - I've added a comment where the potential for abuse on this is discussed on the bitcointalk forums. Let me know what you think.