Extend graph visualizations to also include commit authors

[oindrillac]

In the graph analysis notebook, we consider committers as a contributor. But as per the issue highlighted by James Kunstle here: "Commits have both Authors and Committers who are not necessarily the same person. It certainly isn't incorrect to use only the committer as this data-point, but perhaps there's something interesting to be used later by considering both the 'Committer' and the 'Author' because we'd pay attention to both contributor's work."

We want to extend the data we are collecting to also include authors to make the analysis more complete.

[JamesKunstle]

@oindrillac Is this issue still necessary?