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. I added this support to your library so I could get apples-to-apples results on pycoin sigs vs. my HW wallet sigs. The RFC6979 implementation was lifted from https://github.com/warner/python-ecdsa. 

See the RFC for more details. Let me know what you think.