Adding smooth transition when showing the topbar menu items in small resolution

Right now the menu items from the top-bar shows up abruptly without any transition when revealing it using the Menu button in small resolution. This PR adds a smooth transition using CSS3 animation giving it a smooth transition as such:

![azr5g5umxe](https://cloud.githubusercontent.com/assets/498669/4366950/e4767614-42c9-11e4-91c8-dc4e69c2e802.gif)