Use CSS transforms instead of transitioning positional properties

I'm aware that #15 tried to do the same thing that I'm doing here, but the difference with this pull request is, that it will not break any fixed elements.

In order for the toggle button to stay fixed, I moved it out of the `.container` inside of `.masthead` and placed it directly under the `body`.

I also got rid of the `perspective` properties as they served no obvious purpose to me and I removed some unnecessary vendor prefixes, based on the output of [ai/autoprefixer](https://github.com/ai/autoprefixer).

A preview can be found at http://lanyon.hnrch02.me/