Add ephemeral encryption option to nymserv

These changes add an ephemeral encryption option to nymserv. The configuration header to enable this new option is:

ephemeral: <your ephemeral encryption key here>

Key management for ephemeral keys is done using the Axolotl protocol as implemented in the pyaxo python module from https://github.com/rxcomm/pyaxo. Installation of pyaxo and dependencies is required for the nymserv script to run with the changes in this pull request. The ephemeral encryption key in the configuration header serves as the shared secret between nymserver and client to initialize the key database. A python script to manage client-side encryption keys, create nyms with ephemeral encryption, and use the nymserver (together with some brief documentation) is available for download at http://anemone.mooo.com/nymserver.

Note that all original nymserv configuration options are preserved. These changes only add the ephemeral option.