Fixed #20829 -- skip postgis geometry metadata tables with introspection

Using postgresql-9.2.4 and postgis-2.0.3 the inspectdb tests fail on the geometry metadata tables.

```
ERROR: test_special_column_name_introspection (inspectdb.tests.InspectDBTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/harm/dev/django/tests/inspectdb/tests.py", line 142, in test_special_column_name_introspection
    call_command('inspectdb', stdout=out)
  File "/home/harm/dev/django/django/core/management/__init__.py", line 159, in call_command
    return klass.execute(*args, **defaults)
  File "/home/harm/dev/django/django/core/management/base.py", line 289, in execute
    output = self.handle(*args, **options)
  File "/home/harm/dev/django/django/core/management/base.py", line 419, in handle
    return self.handle_noargs(**options)
  File "/home/harm/dev/django/django/core/management/commands/inspectdb.py", line 27, in handle_noargs
    for line in self.handle_inspection(options):
  File "/home/harm/dev/django/django/core/management/commands/inspectdb.py", line 99, in handle_inspection
    field_type, field_params, field_notes = self.get_field_type(connection, table_name, row)
  File "/home/harm/dev/django/django/contrib/gis/management/commands/inspectdb.py", line 13, in get_field_type
    field_type, geo_params = connection.introspection.get_geometry_type(table_name, geo_col)
  File "/home/harm/dev/django/django/contrib/gis/db/backends/postgis/introspection.py", line 78, in get_geometry_type
    (table_name, geo_col))
Exception: Could not find a geometry or geography column for "raster_columns"."extent"
```
the test fails because the geometry type lookup does not contain metadata about the metadata tables themselves.
https://github.com/django/django/blob/master/django/contrib/gis/db/backends/postgis/introspection.py#L51

postgis itself has models for those tables so a user should never need to create models for them.
https://github.com/django/django/blob/master/django/contrib/gis/db/backends/postgis/models.py