Allow user to specify content he wants to see on project page

Improvement to !938. Since a lot of people complain on fact they don't like to see README.
Especially since we already have a switcher for dashboard - it kind of acceptable to select behaviour.

User can specify which content he/she wants to see:

![Screenshot_2015-07-13_18.24.31](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d237474e8f52df4a62ba7e951fa28bd8/Screenshot_2015-07-13_18.24.31.png)

![Screenshot_2015-07-13_18.24.36](https://gitlab.com/gitlab-org/gitlab-ce/uploads/473b0e2a94b3de16d048b8332b539816/Screenshot_2015-07-13_18.24.36.png)

cc @JobV @jacobvosmaer @marin @sytses

See merge request !970