added logLik method for probit models

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1516 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests for logLik method for probit models

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1517 d36ae590-6c38-44c5-9ba5-7217e93607d8