usage of general jQueryUI Plugins

Enable the use of jQueryUI Plugins that are created by the "widget factory" utility of jQUI (http://api.jqueryui.com/jQuery.widget/) Before, just the widgets that jQueryUI brings with it were supported. 

Changes include: 
- a possible uiNamespace setting on the ember view object. It makes it possible to use plugins that use other namespaces than the jQUeryUI native "ui" one. The "ui" namespace is default, so it's specification is not needed if only native jQUI Widgets are used 
- The retrieval of the ui property of the mixin has been changed in order to be able to retrieve not only the native jQueryUI widget instances. Native ones are defined as properties on the jQuery UI object itself, which was previously used. All plugins created by the widget factory are saved using a data-property (http://api.jqueryui.com/jQuery.widget/  Heading: widget). Now the widget is first created on the jQuery-Object of the views DOM-Element, than the reference to the widget itself is created and saved like before.