Sign then Encrypt

Just as a heads up this is wacky as shit because of some weird merging and rebasing…I'm still looking at it but if you see anything odd please let me know.
@El--Presidente The sender may be exposed in other ways. If we encrypt via pyelliptic, then the sender's public key will be [prepended to the ciphertext](https://github.com/yann2192/pyelliptic/blob/master/pyelliptic/ecc.py#L464), so the sender is revealed.