Added support for \cfrac

This addresses issue https://github.com/Khan/KaTeX/issues/85.

It needs a huxley test.  I've attached some screenshots of ```\cfrac``` vs ```\dfrac```.  I also did some tests with LaTeX to make sure that I was getting the proper spacing.

@xymostech If you happy with the rendering do you want me to try and update the huxley tests?

cfrac:
![screen shot 2014-09-28 at 5 59 24 pm](https://cloud.githubusercontent.com/assets/1044413/4436201/08708de2-476c-11e4-8b7d-98f39bc0d25e.png)

dfrac:
![screen shot 2014-09-28 at 5 59 52 pm](https://cloud.githubusercontent.com/assets/1044413/4436203/13bcb1da-476c-11e4-9574-23fbc61dfa4f.png)

cfrac vs dfrac in LaTeX
![screen shot 2014-09-28 at 6 05 56 pm](https://cloud.githubusercontent.com/assets/1044413/4436211/6acb7ae2-476c-11e4-9a94-376253b38899.png)

This definitely shouldn't be here. When we make the algorithms in here, it's not because it looks good, it's because that's what the TeXbook says to do. For now, we haven't deviated from that, and if we do it would probably be by tweaking some fontMetrics constants. Any time when you tweak things like this, it means that something else is wrong somewhere, and trying to fix it by slightly messing with things will probably just cause more problems in the future.

Do things look actively bad when you don't put this in? Or was this just to make it look like LaTeX does? I'm perfectly fine with having it look different, just as long as it doesn't look bad.