Fix the null attribute bug ([#5931] Crash if attribute is set to NULL != documentation)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/subgroup@52 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Improve API by removing the S4 specific syntax to create a new SDTaskConfig  object . Updated documentation accordingly.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/subgroup@53 edb9625f-4e0d-4859-8d74-9fd3b1da38cb