Phase 1 of SHA256 support
Phase 1 of SHA256 support

- new variable "digestalg" which defaults to "sha1", but allows "sha256"
  for those who want to sign using this
- Addresses #953

hb9kns/PyBitmessage,bmng-dev/PyBitmessage,hb9kns/PyBitmessage,hb9kns/PyBitmessage,bmng-dev/PyBitmessage,bmng-dev/PyBitmessage,hb9kns/PyBitmessage