Added sildeup effect to header with headroom.js


![capture-1400512994](https://cloud.githubusercontent.com/assets/1053079/3016062/bba76e64-df69-11e3-9cdc-e3e3edff84b0.gif)


Adding a smooth scroll effect to hide the header. It brings elements into view when appropriate, and give focus to the content the rest of the time. Transition is of 0.3s, the gif framerate here is low.
__·__ https://github.com/WickyNilliams/headroom.js