Update scoring function.

Before we were incorrectly scoring our background. Using the new
equation:

score = (nb_primers * fg_mean_dist * fg_variance_dist) / (bg_ratio)

where bg_ratio is bg_genome_length / count_of_bg_mers

This will be faster, and will simplify our code (no whcky appends or
anything.

It can be even faster if we want to by not even storing the background
points, just their hits. Should be relatively easy to do that.
start to add debug variable to score_mers
add score wrapper and SelectiveGenomeUI to make install