plot.py: Fix relative score and ordering argument
plot.py: Fix relative score and ordering argument

- Instead of just counting the number of positives and negatives and
  dividing by the total count of nonzero scores, we now weight the counts by
  the relevant scores. We still factor in the group sizes by scaling the
  relative scores by the size of the group divided by the largest group.
  This way small groups do not fudge the results, and larger groups have better
  relative weighing.
- This also removes a hardcoded threshold for the group size which depends on
  the size of the commit comments dump, and with all dumps this is bad.
- Use a DataFrame to store all the parameters of the groups in, simplifying
  the calculations which need the entire results. This also makes the sorting
  somewhat clearer in what column is being sorted.
- Made it possible to give the sort as an argument, by passing the rest of the
  argv to the Plot classes. Thus one can give 0 to set sort on negative values.

timvandermeij/sentiment-analysis,timvandermeij/sentiment-analysis