Updated the Hashtable-Provider for more flexibility 

Added the feature to customize the array-format optionally. e.g. this will now work too: 
```php
new Strong(array(
  'provider' => 'Hashtable',
  'users' => array('username' => array('password' => 'password', 'email' => 'a@b.com'))
));
```

But of course this works out of the box too:
```php
new Strong(array(
  'provider' => 'Hashtable',
  'users' => array('username' => 'password')
)); 
```