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 think I'd rather this argument be called "breakOnInfix", and make the check simply break on infix nodes. Otherwise, if we want to break on something else in the future, we'd either have to make this a list or something.