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 rather if you did a check up here to see if the group is already an ordgroup and if it is, don't wrap it. Otherwise, we're creating ordgroups just to get rid of them (bad for allocations). Also, getting rid of all nested `ordgroup`s is sometimes a problem, since `{a}` does not act the same as `a` in many situations.