Fixed issue #230 -- Avoid queries in aborted transactions

On PostgreSQL when the transaction is in aborted status, checking
connection.isolation_level will result in an error.
Made usable connections with alias not in db.connections
Made possible to use DJANGO_SETTINGS_MODULE in testing

Also added a dummy test_pgsql settings file.
Tests for issue_230
Merge branch 'issue_230' of git://github.com/akaariai/django-debug-toolbar into akaariai-issue_230

Conflicts:
	debug_toolbar/utils/tracking/db.py