boosting: move around documentation
moses: effective length is part of bscorer, not ensemble.

Only the scorer really can relly know how to dal with the
normalization issues of weighting.
moses ensemble: more code mangling.
Move the re-weighting codde into the scorer, out from the ensemble.

The pre scorer is broken, unless this is done.  The pre scorer is
just to different from the regular boolean scorers to have the right
thing happen.