Added code and description for this simple exercise: check social security number validity

Added a simple exercise. See .md file for more details.
To me it does two things with the string `string`:

1. It cleans whitespace and dashes from `string`
2. It converts `string` to a list

The last thing is to me unexpected from using the function. 

And is it necessary to loop through the characters, or is that intentional from a pedagogical point of view? What about the `replace` method for `Strings`:

```python
def clean(string):
    """Clean whitespace and dashes from 'string'"""
    return string.replace("-", "").replace(" ", "")
```