Add instructions gif display option

1. Add ani-gif instructions and preview area display support. Use by setting a `level.aniGifURL` (none set at the moment)
1. Add ability to re-open level instructions by clicking ani-gif preview
1. Add clearfix to instructions area [finishes #72923446]

Still to do:

* Show either (1) pre-baked ani-gif thumbnails or (2) auto-gen thumbnails with grunt or [freeze gif playback](http://freezeframe.chrisantonellis.com/documentation/)
* Tweak layout based on actual ani-gif sizes

Clearfixed single-line instructions:

![image](https://cloud.githubusercontent.com/assets/206973/3294806/dba7f4b0-f5b0-11e3-8056-715632fd8163.png)

Ani-gif preview:

![image](https://cloud.githubusercontent.com/assets/206973/3294671/aca2592e-f5ad-11e3-8f20-f0df2a3ef497.png)

Ani-gif instructions display:

![image](https://cloud.githubusercontent.com/assets/206973/3294672/b433dab4-f5ad-11e3-8f33-64b8d9b6be70.png)
Seems like we use with-ani-gif as both a class name and a suffix (i.e. class="prompt with-ani-gif" and class="instruction-with-ani-gif").