Adds Bower manifest.

This adds a [bower](http://bower.io/) file, if you're interested in supporting Bower. If you are then [you'll want to run](http://bower.io/docs/creating-packages/#register)

```
bower register katex https://github.com/Khan/KaTeX
```

after merging this.

*Note: I already signed the CLA*
This should actually say "Fast math typesetting for the web.", but I forgot to change it in package.json.