Use HealthCheckView to search, load, run and display results of health checks

This pull-request is based on #2.

The feature summary is:

* create some HealthChecks just as you would create TestCases (see #2)
* configure a view that does the same as would "nosetests", i.e. you configure a view to render all HealthCheck we can find in some path.

Example of the "selection" mechanism:

*when you would run the HealthChecks in shell with some `nosetests django_doctor_demo.simple.healthchecks`
* then you would use this in URLs: `django_doctor_demo/simple/healthchecks` (dots are replaced by slashes)

See the demo folder for an example:

* HealthCheck definition: https://github.com/novagile/django-doctor/blob/837187b184ead0591f7ec0f74c7c010ea2488461/demo/django_doctor_demo/simple/healthchecks.py
* URL mapping: https://github.com/novagile/django-doctor/blob/837187b184ead0591f7ec0f74c7c010ea2488461/demo/django_doctor_demo/simple/urls.py
* tests that illustrates how to select HealthChecks via URL: https://github.com/novagile/django-doctor/blob/837187b184ead0591f7ec0f74c7c010ea2488461/demo/django_doctor_demo/simple/tests.py