Add CelerySignalProcessor

Since the private `_setup_save` and `_setup_delete` methods are gone from Haystack's `SearchIndex` the `CelerySearchIndex` won't work anymore.

The new proper way to setup updates is through [Signal Processors](http://django-haystack.readthedocs.org/en/latest/signal_processors.html). This is what this pull request does. It should be backwards compatible to Haystack v1.

Since the signal processor has to decide which models to update, it needs a model configuration with the new setting `CELERY_HAYSTACK_MODELS`.

Unfortunately Haystack 1.X and 2.X will no longer have the same setup. This still needs documentation, but I wanted an opinion about this approach before writing this up.