New API and pep8 cleanup (issue #383)
Make migrations not complain about empty routes
Added <iframe sandbox> style handling of HTML part emails, more to be done #631
Started work on leap plugin.

- Uses new plugin structure.
Removed soledad from mailboxes.
Renamed plugin to leap.
Added startup and shutdown to plugin manifests.
Removed sync from mailbox.unparsed.
Moved config fetch to provider.
Provide mailpile config as param to mailboxes.
Moved LeapSession to session.py.

- Removed unused code.
Added mocking tests for leap integration.
Moved leap session dependency creation to factory.
Started tests on LeapSession.
Moved leap requirements to leap plugin folder.