XCRI parser / fix bugs.

1) characters method should not append to an array as it might be called multiple times (but in the same element)
2) properties after the element xcri:venue were not handled at all - added case in unit tests for that
Merge branch 'master' of github.com:ox-it/moxie-courses

* 'master' of github.com:ox-it/moxie-courses:
  Updated to search for all presentations which don't start in the past