Implemented #55 - PassThroughManagers can have tab-completion/dir() support

While presumably `PassThroughManager` will eventually be subsumed by the `QuerySet`/`Manager` delegation refactor in Django core, this should allow for slightly better usability in current projects (see #55).

PR mostly exists to get a travis run to cover my back. If it passes and no objections get raised after a suitable period, I'll get it merged.