Moved postgresql backend DatabaseOperations class into a new module, postgresql/operations.py, so that it can be imported by both the postgresql and postgresql_psycopg2 backends. Hence the two backends no longer have a duplicated DatabaseOperations class

git-svn-id: http://code.djangoproject.com/svn/django/trunk@5972 bcc190cf-cafb-0310-a4f2-bffc1f526a37