MAHOUT-1596: implement rbind() operator

Signed-off-by: Anand Avati <avati@redhat.com>
i keep a nagging feeling that there got to be a better way than writing something like `implicitly[ClassTag[Int]])`. The fact that it is implicitly inferrable without special context means there's a public `val` defined somewhere for it, so perhaps a better style is to refer to that val explicitly than ask the compiler to find it for us for something that is overtly general type.