Changes to make Extensible ready for other types of calendar

While I was trying to add Jalali calendar to Extensible Calendar I came across couple of changes to the core. As you can see these changes are completely safe and has no affect on the original code. I've also added a locale file for Farsi. To be able actually use the Jalali calendar some changes must also be made to ExtJS's Farsi locale file. Things like Ext.Date.formatFunctions and Ext.Date.parseFunctions which I didn't include here. Perhaps I should send ExtJS project a pull request as well!

To explain the modifications to Extensible.Date class; getDate and getMonth methods of JS's original Date class are moved to Extensible.Date so they can be override in locale file.

There's also a date format added to calendar panel: jumpToDateFormat