new file:   modifiers.py
This has some utility functions, 1) the super simple calculation for bonus/penalty from your ability score and 2) the way to calculate the 'point buy' point value for a set of scores. It may be better to split these two later, as the point value for a set of scores is really only needed for the second post to my Game Balance series at http://corerulebook.com/Game-Balance
	modified:   histograms.py
Added the median calculation for my own edification.