BF: return signed scores when two-sided testing.

This goes with the correction I pushed into master regarding issue #217.

It is important to return signed t-values, even in the case their absolute value has been considered for a two-sided test. It makes the user able to study the effect sign a posteriori.

I added a test to check that the two-sided test actually does recover positive and negative effects. That's indeed what was missing to avoid issue #217 to be introduced into the master branch.

Note that I improved the Oasis example so that the sign of the effect can be observed. I think it is nicer and also the comparison with Anova + SVM is more relevant.