Adds animation to the menu toggle

This pull request resolves #134 in a way that's even better than that Codepen [example](http://cdpn.io/ycudz).
Also fixes this problem:
![image](https://f.cloud.github.com/assets/351568/898744/c5ce8d16-fb1f-11e2-8a8b-21e06804a470.png)

like this:
![image](https://f.cloud.github.com/assets/351568/898747/ec17dc98-fb1f-11e2-9a41-fb9641915ffe.png)