[WIP] Refactor and test crypto_util. Wrap pyelliptic.

I am posting this so that no-one else wastes time doing the same task. Please don't review yet, I 'd like to simplify the code and add a couple of tests first.
This _myself thing survived the factoring of the listener, the refactoring of connection.py and now the refactoring of crypto_util.py. I guess we will remove it in a subsequent PR after the two refactorings get merged?