Fixed contributors updating. Moved resulting html to partial.

I noticed that contributors' icons lead to json document instead of it's user homepage on github.
Then I noticed that rake task for updating contributors is very outdated. Unfortunately `gh_contributors` gem does not allow to provide a template for rendering and newer gem `contributors_stats` requires ruby-2.0.
So in order to fix contributors links I did rendering myself using `gh_contributors` provided data.
Plus I moved the resulting html to `shared/_contributors` partial.

Please, run `rake update:contributors` every time you deploy to keep information up to date.