Update v0.0.4-alpha

* Enhanced the preview function in the backend. The preview link will
now only be displayed, if the page is accessable.
* Changed the backend and frontend layout to allow for a bigger sidebar
and multipe navigation layers.
* Session::printMessage() was renamed to Session::getMessage() and now
returns the message instead of displaying it.
+ Added form and error styles for the frontend layout.
+ Added a contact form plugin.
+ Added the method Router::here() to return the current pages url
+ Added a proper display of the 404 error