Improved Calendar Support

Fixes previous CalDAV support, as MobileMe based calendars
now work ok. Additionally, a new 'id' field has been created
to provide a persistent and immutable identifier for a ride
file (regardless of changes to date/filename).

The URL provided in the Calendar config pane can now include
'@' symbols (they are translated to %40).

The CalDAV URL should be provided for a calendar collection and
not for a principal. Examples being (trailing slash is significant):

Google  : https://www.google.com/calendar/dav/xxxx@gmail.com/events/
MobileMe: https://cal.me.com:8443/calendars/users/x.xxxxxxxxxx/home/

src/MainWindow.cpp

@@ -320,7 +320,7 @@ MainWindow::MainWindow(const QDir &home) :
 #ifdef GC_HAVE_ICAL
     rideCalendar = new ICalendar(this); // my local/remote calendar entries
     davCalendar = new CalDAV(this); // remote caldav
-    davCalendar->download(); // login
+    davCalendar->download(); // refresh the diary window
 #endif
 
     QTreeWidgetItem *last = NULL;