Spruce up priming view logic in import

Prevent back to back priming (giant slowdown) and use actual changes to
data to decide when we've processed enough entries to fire the cleanup
Add trailing slash to url
Move prime view method to dimagi-utils
Merge pull request #382 from dimagi/celery

Move case importer work to celery
Correct import for importer exception types
Merge branch 'case-importer' into staging2