Allow HAYSTACK_SIGNAL_PROCESSOR Class to Import Models

Currently, if you use the HAYSTACK_SIGNAL_PROCESSOR function to set to a class which imports a model, then you can get ciruclar imports. This was also reported here:
http://stackoverflow.com/questions/17049426/importerror-when-using-haystack-2-0-0-with-django-1-5-and-gunicorn-wsgi

It would be nice if our signal processors could import our models.

I think the issue is in __init__.py here:

# Setup the signal processor.
signal_processor_path = getattr(settings, 'HAYSTACK_SIGNAL_PROCESSOR', 
   'haystack.signals.BaseSignalProcessor')
signal_processor_class = loading.import_class(signal_processor_path)
signal_processor = signal_processor_class(connections, connection_router)

We should not be importing or instantiating this class within __init__.py, otherwise it can easily cause circular imports. Instead, we should try to lazy load it.

Start in init.py at the signal processor setup and read how loading.import_class and instantiation are performed. Reproduce the configuration with a HAYSTACK_SIGNAL_PROCESSOR class that imports a model, then trace the circular import. Done means signal processors can import models without causing circular imports during initialization.