Show length of issue in ical

[mAPBhlJ]

If you have an issue that spans multiple days, it'll only show up on the last day of the issue.
(as per readme->features), but I believe it would be more useful to show the spanning issues as a spanning task in iCal.

So that startdate of the event is matched with the issues' startdate
And the enddate of the event is matched with the issues' enddate.
Instead of both being matched with the issues' enddate.

So instead of:

| Jan | Feb | March |
|

It should be:

| Jan | Feb | March |
|<----Issue 12---->|

in iCal.