Correctly incorporate committer data everywhere

[bockthom]

As we now have committer data available (#35), we need to incorporate the committer data everywhere:

• Rename several functions and configuration options (e.g., author.only.committers is not correctly named any more)
• Provide access to aggregated committer data (e.g., committer2artifact (respect different kinds of artifacts), committer2commit, etc.)
• Enable constructing committer networks (similar to all the other author networks)
• Construct networks with authors and committers (possible for author networks, bipartite networks, multi-networks)

Anything to add here?

[bockthom]

• Also think about about author-role classification using committer networks (not author networks) or committer commit count (instead of author commit count). See here.