Add AUTOCOMMIT isolation level support for psycopg2

One can use this to emit statements, which can not be
executed within a transaction (e. g. CREATE DATABASE):

    from sqlalchemy import create_engine

    eng = create_engine('postgresql://test:test@localhost/test')

    conn = eng.connect().execution_options(isolation_level='AUTOCOMMIT')
    conn.execute('CREATE DATABASE test2;')

Fixes issue #2072.