Intersection tables: make DB name a parameter
Intersection tables: handle all three orientations in PL/pgSQL function
Intersection tables: remove debug output
Move tests into own subdirectory

Also the name of the test module changes to let it be found by Django's test
discovery (all files matching test*.py).
Intersection tables: add table creation test case
WIP: test case
Add missing user and project_stack data to test data
Intersection tables: add basic test cases for PL/pgSQL functions

There is one test case to test if the new table is actually created in
the expected format and the one tests tests the intersections. The tests
can be run with this:

./manage.py test catmaid.tests.test_plpgsql