Added date.js as fallback include. 

When using the en_US locale date.js will not be loaded in yaft.js since the locale from sakai will be empty where it tries to load. To trigger an error that this will solve, set language preference to en_US, create a forum with visibility limitations, save forum, then try to edit the forum, yaft.js:885 will throw an exception since date.js has not been loaded. jquery.ui.datepicker.js already uses this strategy.