Import signal receivers in __init__.py (not models.py)

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)