Use zc.recipe.egg to create wrapper scripts

This requires a part like 

    [scripts]
    recipe = zc.recipe.egg
    # Generate console_scripts
    eggs = makina.recipe.postgres
    arguments = '${postgres:location}/bin', '${config:postgres_data_dir}', '${config:postgres_db_name}', '${config:postgres_superuser_name}', '${config:postgres_port}'

to generate and initialize the scripts. 
It also assumes we're using a template recipe to create a postgres.conf 
so doesn't attempt to set `port` in that file.