added support for \over

This addresses issue https://github.com/Khan/KaTeX/issues/74.

The parser rewrites \over as a ```\frac```.  ```1 + 2i \over 3 + 4i``` -> ```\frac{1+2i}{3+4i}``` as a post processing step in ```handleExpressionBody```.

There's unit tests too!  ^_^