Extend the XP type system

This pull request implements xp-framework/rfc#266 and implements:

* [x] Wildcard types
* [x] Type::newInstance()
* [x] Type::cast()
* [x] Type::$Default

## Example

This can be useful when handling types in a generic fashion:

```php
class Storage extends \lang\Object {

  /**
   * Get an entry
   *
   * @param  lang.Type $t
   * @param  string $name
   * @return var An instance of the given type `$t`
   */
  public function entry(Type $t, $name) {
    return $t->cast($this->read($name));
  }

  // ...
}

$entry= $storage->entry(Type::forName('de.thekid.dialog.Album'), $req->getQueryString());
```

Together with generic methods (see xp-framework/rfc#214) this could be rewritten even shorter.,