Accept more than 10 TS/DL uid

When the org exporter add an TS or DL prefix to an uid, it can an a
if there is more than one. This number can be greater than 9; making
org-caldav to fail to sync. Let strip as many digit as there is.

Signed-off-by: Rémi Vanicat <vanicat@debian.org>
Merge pull request #34 from vanicat/t/prefix

Accept more than 10 TS/DL uid