Enable travis ci

Enable travis ci for continuous integration tests.
 - add .travis.yml
 - add config.xml for unit tests
 - fix some unit tests
 - don't require values in unit test database anymore
 - remove update files for single revisions

Bug fixes:
 - don't require user-plugin in class::UserData
 - add missing 'category' to structure.sql
 - json importer: fix uncommented lines