Remove useless subtract_neg_target from precision

Hardcoded to true and added 0.5 to the bscore to keep same score, like
in prerec_bscore.
Temporarily disable all moses utests except the one failing, test_precision()
Fix precision_bscore::sum_outputs (subtracting unsigned wasn't good)
Re-enabling all unit tests of MOSESUTest
Remove bscore output (and add comment to remind the badness of it)
Merge pull request #195 from ngeiswei/simplify_precision_bscore

Simplify precision bscore