make sure the response can be missing in newdata

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@653 255f4249-2263-4417-b1ea-8498779e27a7
add levelplot of conditional densities to example(cforest)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@654 255f4249-2263-4417-b1ea-8498779e27a7