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.
Hmm, these got moved too far out. My rule is "move variable declarations to the outermost scope that they are used" (even though javascript semantics don't care). So these should be put inside the if statement on line 170 since they aren't used outside.