Moses: extend _perm_ratio in backwards-compat way.

It seems that the old code (a few months ago) build significantly
fewer pairs than the current code, and this strongly affects scores
and learning.  So extend the meaning of _perm_ratio in a backwards-
compatible way, so that this can be explicitly controlled and tested.
Merge pull request #200 from linas/moses

Moses: extend _perm_ratio in backwards-compat way.