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)

The `amsmath.sty` definition of cfrac is:

```
\newcommand{\cfrac}[3][c]{{\displaystyle\frac{%
  \strut\ifx r#1\hfill\fi#2\ifx l#1\hfill\fi}{#3}}%
  \kern-\nulldelimiterspace}
```

I'm not the world's best TeXnician though so I can't say I'm sure what exactly that means.