Fix error on CLASS:X-PERSONAL

Summary:
This X-* property is standard, but not supported by Kolab format.
Though we should not bail out, we should just ignore the value not the whole iTip.

Reviewers: PyKolab Developers, vanmeeuwen

Reviewed By: PyKolab Developers, vanmeeuwen

Subscribers: vanmeeuwen

Differential Revision: https://git.kolab.org/D479