added VALARM to birthday events

I did this small change for my owncloud installation, because I want to get a reminder on my mobile for contact birthdays.

Some devices might generate audible ALARMs at 0 o'clock from them, despite the VCALENDAR entry saying it should only display something. So it might be a good idea to make this optional or the offset configurable.. for example changing the trigger to PT12H would display the alarm at noon. 
Currently I can't see any options for the contacts app, so maybe this is not feasible either. 

Anyway, for me this works fine and I wanted to offer this change to you.