Avoid Sabre version conflicts with other plugins

Hi Benjamin,

Thanks very much creating rcmcarddav plugin. I used it together with the calendar plugin created by kolab. However, the plugin uses a different version of Sabre which causes the following errors. 

`Got error 'PHP message: PHP Fatal error:  Class vobject_location_property contains 3 abstract methods and must therefore be declared abstract or implement the remaining methods (Sabre\\VObject\\Property::getValueType, Sabre\\VObject\\Property::setRawMimeDirValue, Sabre\\VObject\\Property::getRawMimeDirValue) in /var/www/html/rcb/plugins/libcalendaring/libvcalendar.php on line 1121\n'`

So I think it is better to load the required libraries selectively in accordance with necessary tasks.

Regards,
Charlie