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
Why not create this message in the for loop instead of creating a `lambda` here?

```python
for validator in validators:
    if not validator(output):
        msg = 'Validator {0}({1}) is not True'.format(validator.__name__, output)
        raise exception_class(self.error or msg)
```