Issue/#34 - Passing a list of callables for validation

Added the possiblity to pass a list of callable for validation as asked here: https://github.com/sloria/marshmallow/issues/34
Since this example isn't really an improvement of the previous example, I think this code example can be replaced with a simple note after the previous example.

```
.. note::

    You may also pass a list of callables to the ``validate`` parameter.
```

I would like to keep the Quickstart guide as concise as possible without sacrificing completeness.