Use all indexes that the connection router returns

Connection router returns a list of backends
Add CelerySignalProcessor

The new way for haystack to notify updates are
signal processors. Since they get post_save and
post_delete signals from all models, we have to
filter here with a CELERY_HAYSTACK_MODELS setting.
Add Django 1.5 to travis build matrix
Determine right index without setting

- removes the setting
- checks all backends
- checks if index is instance of CelerySearchIndex
Merge branch 'develop' into signal-processor

Conflicts:
	.travis.yml
Check should_update of index before enqueueing task
Remove obsolete import
Merge pull request #10 from stefanw/signal-processor

Add CelerySignalProcessor