[Calendar] - Remove forced plug of CalendarNavigator and updated navigation control templates

The Calendar widget forces the plug of CalendarNavigator in the initializer, which breaks the purpose of a plugin being an instance-level enhancement/ augmentation. With this modification, I have added a navigatorPlugin attribute that developers can use to specify a navigator without having to unplug the forced CalendarNavigator. Of course, the default value is Y.Plugin.CalendarNavigator.

Along with this I have updated the CalendarNavigator prev/ next month controls to use buttons instead of anchor tags for better semantics and accessibility. CSS updated to support this.

Finally, I externalized the CalendarNavigator's aria-label strings using the Y.Intl utility by adding calendarnavigator.js and calendarnavigator_en.js to the lang directory.

Updates to the apidocs have been done, and I tested these changes on Win 7 with the following browsers: IE8, Chrome 26.0.1410.64, Firefox 20.0.1, Safari 5.1.7. Updates to unit tests not necessary.