flux:flexform.field.userFunc attribute missing

Hi,

Claus wanted me to recreate the pull request i made :-) ... here it is:

i just wanted to inform you, that you got an bug with `<flux:flexform.field.userFunc>` . The attribute `arguments` is not registered. Just add two lines in that viewhelper and it works with arguments. 

````php
public function initializeArguments() {
		parent::initializeArguments();
		$this->registerArgument('userFunc', 'string', 'Classname->function notation of UserFunc to be called, example "Tx_Myext_Configuration_FlexForms_MyField->renderField" - Extbase classes need autoload registry for this', TRUE);
		$this->registerArgument('arguments', 'string', 'Optional array of arguments to pass to the UserFunction building this field', FALSE,NULL);
	}

	/**
	 * Render method
	 * @return array
	 */
	public function renderConfiguration() {
		$config = $this->getBaseConfig();
		$config['type'] = 'user';
		$config['userFunc'] = $this->arguments['userFunc'];
		$config['arguments'] = $this->arguments['arguments'];
		return $config;
	}
````