Database independent testing query

[jor-rit]

Replace the current 'SELECT 1' query, with a database independent solution. 'SELECT 1' does not work on backends like Oracle and DB2.

This is a follow-up on #8.

Proposal

Django's database API has an is_usable method, implemented by all backends that abstracts away this query (for example it does SELECT 1 query on PostgreSQL and connection.ping() on MySQL).
So let's reuse that. Means getting the backend from django.db.connections, creating a connection, and calling the is_usable() method.

Using django.db.connections would also be a first step to being implementing #4.