This reverts fa1f8403 which moved them from models.py to __init__.py.
As reported by @mvantellingen, importing receivers from __init__.py
breaks the way models are normally overridden as it causes the core
models.py to get imported before the custom one.

We need to move back to the old system and will need to advise that the
debug toolbar needs to be set-up explicitly to avoid circular import
issues with models.

Fixes #1159
Original issue #1125

This is to avoid circular import issues as most receivers.py modules
also import models.  It's not entirely clear what circumstances are
required to get the circular import - it seems to be when using
django-debug-toolbar>=1.0, which changes the way the URLs are
monkey-patched.

This changes adopts the recommended practice for importing signal
receivers:

- https://groups.google.com/forum/#!msg/django-developers/PReHoxwwG_c/M-asj72ks7YJ
- http://stackoverflow.com/a/14192257/25517

Fixes #1125 (I think)

Analytics allows tracking of user and product activity.