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)

Here's what the TeXbook has to say about ```\strut``` (page 82):

<em>Plain TEX defines \strut to be an invisible box of width zero that extends just enough above and below the baseline so that you would need no interline glue at all if every line contained a strut. (Baselines are 12pt apart in plain TEX; it turns out that \strut is a vertical rule, 8.5pt high and 3.5pt deep and 0pt wide.)</em>

Does that that there should be a (8.5/12) 0.708333 em space above the baseline and a (3.5/12) 0.291666 em space below?