Commit calendar for the user profile

What does this MR do?

This MR adds a commit calendar to the profile page of a user. When user has commits, it will show a calendar covering a timeframe from the year before up to his latest commit.
Currently the calendar only counts the commits. It does not show which commits were actually made.
I plan to add that in a later MR at some point in the future.

Are there points in the code the reviewer needs to double check?

I guess test coverage (definatly lacking) and maybe the variables in the user_controller. I'm not sure if they really need to be instance variables.
Also probably the repository.rb.
Maybe gitlab introduced some other methods to achive the same thing easier already :)

Why was this MR needed?

It's always nice to see how much a person contributed to a project on a single glance :)

What are the relevant issue numbers / Feature requests?

This MR implements http://feedback.gitlab.com/forums/176466-general/suggestions/5855106-calendar-of-commits
There already was a MR for this on github, but it got closed because of inactivity. https://github.com/gitlabhq/gitlabhq/pull/7038

Screenshots

@dblessing Can you please take a look at this? :)