Add spoilers functionality

Now all solutions are hidden and must be shown

- Used _templates/layout.html to added a javascript file
- Used _static/spoilers.js that will select all code boxes
after a h3 containing 'Solution' and hide them, with a show button.

Example: http://embed.plnkr.co/fR7Sd6DCQ3Jxzimjvfqj/preview