Responsive navbar

Some fixes to make the topbar responsive
![screen shot 2014-12-21 at 2 33 11 am](https://cloud.githubusercontent.com/assets/1312687/5517663/df27693a-88b9-11e4-9738-db97ce608f03.png)

I changed the signup button when it's on the menu because the button looked weird.
Related to #223