XML feed for ICPC Event Feed standard.

Can be used for scoreboard resolver, MyICPC etc.
As found in https://gist.github.com/meisterT/409cb8cb6b791dae8fd9

See also issue #74
Update for changes in DOMjudge.
Fixes for import-RESTfeed.

Adapt for changes in DOMjudge. More verbose error reporting. Match teams
on their external ID.