sampleSelection now *imports* rather than *depends* on the systemfit package

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1469 d36ae590-6c38-44c5-9ba5-7217e93607d8
added test script (copied from an example in selection.Rd)  and the corresponding output file that test the heckit function with instrumental variables: unfortunately, it seems that this does not work as the results with and without instrumental variables are the same :-(

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1470 d36ae590-6c38-44c5-9ba5-7217e93607d8