cosmetic cleanup for precision bascore
Fix worst-possible-score for the precision scorer.
partial fix for boosted precison scorer.
yikes, typoe in the last commit
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.
Argh.
Fix the selection scorer so that it works again.
Merge pull request #1039 from linas/preboost

Moses: re-organize the boosting code