Multi-View staring an all new architecture with WRetriever and WResponse

WityCMS architecture completely changed in this pull request.
Multi-app rendering is now enabled. Several applications can be displayed on the same final response.

WRetriever is the key class to make the bridge between applications.
This class offers a new WTemplate handler named retrieve_view to get the view of another application. Using this handler, you are able to input new app parameters + GET variables:
```{retrieve_view app_name/action/param1/param2?var1=value1&var2=value2}```

Looking forward to have your feedback on what's good, to be improved, what's missing...