Correct Sphinx syntax in shipping docs

Underscore does not seem to have meaning in Sphinx, was rendered as-is.
Tests and fix for BrowsableProductManager

The changes in 10bfa701 renamed get_query_set to get_queryset, which is
a change that only happened with Django 1.6.

A simple alias fixes the issue. Also added some tests for the manager.

Fixes #1294
Test making demo site in Travis

Ensures that we catch errors in the build process.
Create PostGIS extension in Travis
Correct demo DATABASES settings
Install PostGIS

As found in
https://github.com/charettes/django-mutant/blob/master/.travis.yml
Trial forcing POSTGIS version
Use username known to work with Travis

Docs say "root" should work, but it doesnt.
Work around django extensions lacking PostGIS support

I suspect reset_db might not be working because of it.
Undo unnecessary changes
Use more generic username