Allow choosing normalization mode

According to Dr. Obvious, "different voting modules create different score distributions". Hence, the best normalization mode might be different as well, e.g. log(score)/log(max) for Plus1 (concentration at the bottom) but pow(score)/pow(max) for Fivestar (concentration often at the top).

As we're agnostic to the modules used, we could let the users [who care] choose the normalization mode.

Implementing this would however most likely require re-theming node_search_admin().