Rails 4.2 update

# Do not merge this PR yet.

This pull request should document activities to the next ruby on rails major release (v.4.2.0). The rails-4.2-branch in currently on v4.2.0.beta4. There are still issues and some failing tests.

## Known Issues

- [ ] maybe `Model.reflections()`-calls needs to be updated. See rails/rails#16928
- [ ] remove `extend ActiveModel::Naming` in UserSession-Model after binarylogic/authlogic#432 was fixed
- [ ] There is a problem with job processing. Job-workers won't process pending jobs when using mysql >5.6.4 (see rails/rails#17239 and collectiveidea/delayed_job_active_record/95)
- [ ] use ActiveJob-API and DelayedJobQueingAdaper instead of using DelayedJob directly (optional)
- [ ] replace better errors with new web console gem (optional)