Fixes for Rails 3 and compatibility issues with Redmine 2.4.x

- fixes for Rails 3 (old Rails 2 code) and compatibility to Redmine 2.4 (routes)
- fixed not working creation of my-todos due to bad hard coded relative Redmine url
- fixed bug with plugin settings page
- fixed German translation
- OPEN: sorting/ordering items via JavaScript not working anymore with Rails 3