Add support for forcing input to upper or lower case

This patch adds two new settings: `forceUpper` and `forceLower`. This enables input to be converted to upper or lower case, instead of preventing users from entering upper case or lower case letters as with the existing `allowUpper` and `allowLower` settings.

For example, we want all usernames to be entered in lower case:

```javascript
$('#username').alphanum({
    allowSpace: false,
    allowUpper: true,
    allowLower: true,
    forceLower: true
});
```

Now, if a user types a capital __A__ it will be changed to a small __a__, instead of being silently discarded.