melt enhancements

+ better warnings for certain cases.
+ returns correct "molten" result when nrow=0 now.
+ Raised a deep-if in dealing with 'na.rm' argument - feels good :)
+ better handling of STRSXPs without affecting generation count - should result in slight speed improvement as well.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1199 edb9625f-4e0d-4859-8d74-9fd3b1da38cb