Adapt smtp-relay to use leap.common.keymanager

* Add docstrings to smtp-relay classes.
* Setup test suite.
    * Add setuptools-trial as dependency for tests.
    * Move smtp-relay tests to default test directory.
* Add tests for smtp-relay.
    * Send of unencrypted mail depending on 'encrypted_only' param.
    * Malformed email address.
* Add a helper function for smtp-relay.
* Assert params for initializing SMTPFactory.
* Use mail.util.parseaddr to parse email address.
* Use email.message.Message to represent an email message in smtp-relay.
* Make requirements effective and fix leap.common version in setup.py.
* Add/remove dependencies in setup.py.
* Fix Soledad instantiation in tests.
* Fix sender address in smtp-relay.
* Fix some comments regarding twisted's SSL and SMTP.
* Remove authentication from smtp-relay when sending.

This closes #2446.
Make smtp-relay sign all outgoing message.

* Update docstrings.
* Remove prepareHeader and its bug.
* Make smtp-relay always sign outgoing mail.
Add changes file.