TestCase like health checks

This pull-request is based on #1.

Introduced a HealthCheck class, which is simply a subclass of unittest.TestCase with a is_healthcheck attribute set to True.

Added a sample healthcheck in demo project: https://github.com/novagile/django-doctor/blob/f4fbbb7ce0f702043032c77bb62e223b93a31df7/demo/django_doctor_demo/healthchecks.py (yes, it is really simple for now)

Added `make healthcheck` feature: https://github.com/novagile/django-doctor/blob/f4fbbb7ce0f702043032c77bb62e223b93a31df7/Makefile#L70

And modified `make test` so that it skips health checks: https://github.com/novagile/django-doctor/blob/f4fbbb7ce0f702043032c77bb62e223b93a31df7/demo/django_doctor_demo/settings.py#L77