Implement GET /api/v1/checks/ for listing existing checks

Don't show ping URLs -- gmail converts them to clickable links, and people click on them.

Using backend name django.db.backends.postgresql

Using new backend name which come with Django 1.9 django.db.backends.postgresql instead of django.db.backends.postgresql_psycopg2

Change github URL from SSH to HTTPS in installation instructions.

Add VictorOps integration

Use X-Forwarded-For and X-Forwarded-Proto headers, instead of X-Real-IP and X-Scheme

Add healthcheck UA to inform server about identity

test fix