added support for \over

Implements feature requested in issue https://github.com/Khan/KaTeX/issues/74.  It converts the infix operator \over to a \frac, examples:
- 1 \over 2 -> \frac{1}{2}
- 1+2i \over 3-4i -> \frac{1+2i}{3-4i}
- \displaystyle 2 \over 3 -> \frac{\displaystyle 2}{3}  @spicyj: thanks for finding this bug

It handles the \over in a general enough way that we can add other infix operators by simply adding an entry to functions.js.
I'd be down for either. Not sure if they're flexible enough to accommodate our exact style rules around spacing.