Added Cluster.toString() for debugging purposes.
CARROT-1013: Multilingual clustering and FLATTEN_ALL results in a random
(hash-dependent) order of clusters after merging. This fix also removes
hacky reorderings upon FLATTEN_ALL that were scattered in individual
algorithms. FLATTEN_ALL has a clear sorting policy of reversed-size. If
there's only one language in the result set (which is typically true
because Carrot2 doesn't have language detection built-in) the order will
reflect the original algorithm's order. For STC and Lingo this means it's
adjustable via scoreWeight.