Navbar hides initial content when jumping to in-page anchor

Hi,

First, thanks for the awesome theme! It's really beautiful and efficient.

I've realised that there is the same issue than the one described in https://github.com/twbs/bootstrap/issues/1768.

I don't know if it's intended or not. Anyways, I've copied their solution and added it to your theme.
Yet, I don't get it why I can't retrieve the height from the navbar. Thus in my solution I directly use the raw value in the code.

```
// var navHeight = $(".navbar").height(),
 var navHeight = 40,
```

Let me know what you think.

Best regards,
Pierre