73 pages, 1 figure. Version 2 contains several improvements, in
  particular concerning the proofs of Lem. 2.15 and of Prop. 4.7, and the
  discussion in Appendix A2. Some notation has also been changed. Version 3
  comes with an extended discussion of groupoid bundles with connection.
  Version 4 is the published version. In Version 5 we have replaced parts of
  the proof of Prop. 4.7 by better arguments