MAHOUT-1596: implement rbind() operator

Signed-off-by: Anand Avati <avati@redhat.com>
There really should be only two types of routines:  (1) Int (2) everything else. 

Long does not have ordering semantics (unlike int) because ordering algebraically is only important for transpositions, and since vector.set(i) only accepts an Int, there is no special significance for Long keys thru-out (i.e. log labels are bearing no special meaning compared to, say, 128 bit hashes). 

Other than that, it makes sense. One thing I am not sure if union() is the best operation here, maybe there's a special opertaion that implies just dumping all partitions together without doing reshuffling, need to check with Spark API.