Refactor signals of tasks, userstories and issues modules
:ghost: Fix a strange behavior with get_model in taiga.projects.history.services
Change django.db.models.loading.get_model (deprecated in django 1.7) to django.apps.apps.get_model
Merge pull request #72 from taigaio/refactor/signals

Refactor signals of tasks, userstories and issues modules (and fix bug #1061)