Update argument to serialize all objects

The `*` argument to select all objects created in [Issue 10](https://github.com/davedash/django-fixture-magic/pull/10) does not quite work correctly. In the console, `*` is an alias to select all the files in the current directory. To actually select all objects---and not provide the `dump_object` command with a list of files---you had to use `'*'` or  `"*"`. I changed the parameter to `all`, which seemed more intuitive than `"*"`