Raise error if taggit.migrations imported for Django <=1.6

Fixes gh-219.

taggit/migrations/__init__.py

+"""
+Django migrations for taggit app
+
+This package does not contain South migrations.  South migrations can be found
+in the ``south_migrations`` package.
+"""
+
+SOUTH_ERROR_MESSAGE = """\n
+For South support, customize the SOUTH_MIGRATION_MODULES setting like so:
+
+    SOUTH_MIGRATION_MODULES = {
+        'taggit': 'taggit.south_migrations',
+    }
+"""
+
+# Ensure the user is not using Django 1.6 or below with South
+try:
+    from django.db import migrations  # noqa
+except ImportError:
+    from django.core.exceptions import ImproperlyConfigured
+    raise ImproperlyConfigured(SOUTH_ERROR_MESSAGE)