LAST-MODIFIED not updated correctly

[georgehrke]

see #657

[georgehrke]

I know 100% that we had this one in an earlier release of the new calendar, but apparently it was removed by accident when refactoring some code

[georgehrke]

Solution:
Add touch method to the VEventModel

[georgehrke]

DTSTAMP needs to be updated as well.

From Section 3.8.7.2

In the case of an iCalendar object that specifies a "METHOD" property,
this property specifies the date and time that the instance of the iCalendar object was created.
In the case of an iCalendar object that doesn't specify a "METHOD" property,
this property specifies the date and time that the information associated with
the calendar component was last revised in the calendar store.