Animation of Sidebar (video in here)

Edited the CSS of header and sidebar; added a new div of gitlab-logo-container to the html to make things easier.

The transition-duration is set to 0.3s and when the nav bar expands, it opens up over the text, essentially revealing hidden text.

[animation-sidebar-2.mov](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b15996a256d6bed03a19c68549d116e4/animation-sidebar-2.mov)

/cc @JobV @nicolas2

See merge request !849