MAHOUT-1596: implement rbind() operator

Signed-off-by: Anand Avati <avati@redhat.com>
hm. i wonder if we can just insert mapBlock() operator at logical layer to handle key mutations here on one of the operands and have same semantics (concatenate partitions) at the physical layer. 

Generally, if we can handle a problem at logical level, we always should try to do that, since it minimizes the back-side problem.