first pass at a small notebook for graphing top senders in a list

As a separate notebook, some steps for looking at the senders of messages in a list.

This notebook just creates a chart of the top senders and their number of messages for a single small list, but it's a place to start. I would likely add next: 
* charts of the distribution of senders
* code for guessing / explicitly combining addresses that represent the same person

I'm not crazy about how clean this code is. It feels natural to me to turn a multi-dimensional array into a list of objects/dictionaries with named fields, but then I immediately turn around and turn it back into lists, because that's what matplotlib requires. (It might be that using pandas visualization, we could use DataFrames/Series which have named columns and allow you to plot them without too much array conversion.)