More organization related metrics

[mhuin]

When graphing commits per organization, il would be interesting to display also some metrics on the organization itself, for example:

• the evolution of the size of the org in time (to simplify things, the amount of members at the beginning of each time slice of the commit graph)
• the normalized commit velocity of the organization (number of commits / amount of members/time period)

There should be a toggle button to also display the amount of commits as stacked bars, each bar color representing a specific member of the organization. For very large organizations, it could be limited to top 10 committers (in volume) + all other members aggregated together.