- added and updated references
  (for stability selection, confidence intervals and constrained regression)
- fixed CITATION
- updated stabsel so that a significance level is returned
  (when using stabs >= 0.2-0)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/mboost@807 edb9625f-4e0d-4859-8d74-9fd3b1da38cb