Class validators

As discussed in #85, these functions should be marked for deprecation You could use a decorator, like so:

```python
import functools

def _deprecated(func):
    @functools.wraps(func)
    def new_func(*args, **kwargs):
        warnings.warn('{0} is deprecated. Use the equivalent '
                      'class-based validator instead.'.format(func.__name__),
                      category=DeprecationWarning)
        return func(*args, **kwargs)
    return new_func
```