Add onion model analysis

This analysis provides information about the general structure of the community.
This is divided into core, regular and ocassional authors
 - Core developers are defined as those performing up to a 80% of the total commits
 - Regular developers are defind as those performing from the 80% to a 99% of the total commits
 - Occasional developers are defined as those performing  from the 99% to the 100% of the commits