remove dtend from th xCal export

xCal is a direct port of iCal to XML.
iCal only allows one of dtend and duration to be specified.
The non-normative relax-ng schema in the relevant rfc reflects this.

I removed the duration and not the dtend, as parsers have to support the
datetime format anyways and there are parsers without support for the
duration format.

xCal [rfc 6321](https://tools.ietf.org/html/rfc6321#page-32):

    ```
    (property-dtend | property-duration)? &
    ```

iCal [rfc 5545](https://tools.ietf.org/html/rfc5545#page52):

    ```
    ; Either 'dtend' or 'duration' MAY appear in
    ; a 'eventprop', but 'dtend' and 'duration'
    ; MUST NOT occur in the same 'eventprop'.
    ;
    dtend / duration /
    ```
insert dtend instead of duration in iCal export

There are iCal parsers without support for duration values.
As dtend has the same format as dtstart, parsers which don't support it
have no chance anyways.
Merge pull request #5 from t-8ch/calendar_exports

Corrections to formats of calendar exports for standards compliance