New Layout class for easier constructing of views

Added Layout class to unify header and footer, and clearly show where body should be rendered. Added loadView method for Controller to easily load views using layout path and body path, and added viewBag object to pass variables between controller and view.

Sample load view from controller
$this->loadView('application/views/shared/layout.php', 'application/views/songs/index.php');

Sample layout.php
html
body
  <?php $this->renderBody(); // Loads bodyPath ?>
/body
/html