Create `importer.models` package

Making `species.py` and `trees.py` siblings of `models.py` turns out to have broken South's ability to detect schema migrations in those files.

Initially when splitting up the monolithic `models.py` I had wanted to make a `models` package but ran into migration problems.
Now I've solved those by a migration removing all models (0002) and another adding them back in from the package (0003).
That's OK since nobody has important importer data currently.