Default stats dir and some UI improvements

 * Add default azkaban.stats.dir property and create directory if it does not exist.
 * Clean up CSS styles for flow job list.
 * Change Auto Pan Zoom checkbox into toggle button.
 * Print cause of InvocationTargetException when trying to load viewer plugin so that plugins that cannot load due to an exception thrown by the constructor can be distinguished from the case where the plugin class does not inherit from AbstractAzkabanServlet.
 * Improve appearance of main page flows list.