Refactor Aggregator

Pull parts about splitting by category into a Splitter.
Splitter splits a list of data points into a list of categories.
Add un-minified d3 to ease debugging
Remove styles that are not used
Support multiple series in line chart.

This allows you to visualize commits per dev per day.
It works using Splitter and Aggregator. First split by developer, then
within each developer, aggregate by Day.