Added public user index

The index is analogous to the public project index, except that all users are public.

The public area now has a navigation panel with currently two items: public projects and public groups.

![public-user-screenshot](https://f.cloud.github.com/assets/1429315/1894906/b7ac3538-7b05-11e3-8192-d7a22f986c78.png)

The main use case for now is to find the profile of someone which you only know the name, for example to view his projects. In the future, other things could be added, such as user metrics/analytics.

I am sorry for having multiple commits, but I merged the upstream while developing the feature and now I don't know how to squash them. Next time I will rebase to the upstream instead of merging with it.