Removed ignore_exists function again as it causes pain, fixes #95

Unittests run with new PostgreSQL 9.1 transaction behavior fail
because ignore_exists does not roll back properly but leaves
the database transaction in a bad state.

Besides that it is documented([1]) how to upgrade if this error
occurs.  There is no reason to workaround this in such a way.

[1]: in http://packages.python.org/django-celery/changelog.html#upgrading-for-south-users
Adds Christopher Grebs to AUTHORS