Fix API call descriptions resizing to a smaller width while showing or hiding

This commit provides a fix for a fairly visible bug.
While dropping down an API call description, the description is sized to a smaller width than it should be, and then abruptly resized to the correct width after the animation finishes.
Here's a slowed-down animation of the bug in action:

![preupdate](https://f.cloud.github.com/assets/3643764/2094493/dc71f3b8-8ecd-11e3-9752-ade89e6cf4c2.gif)

After a few minor changes to assets.css, the effect looks much more fluid:

![postupdate](https://f.cloud.github.com/assets/3643764/2094509/26aa7266-8ece-11e3-8dbb-4b5b631f11c4.gif)

I also made one or two smaller changes that many users should find helpful—index.html should now render nicely on smaller-screen devices (it was unnavigable before), and the Open Sans font is now loaded over https, so accessing an API description over https doesn't show any warnings.

(And on a non-pull-request-related-note— this is the best API documentation website that I've seen yet; thanks! :+1:)