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... looking at eslint's rules, the extra space would not have raised a warning.  that being said there are other things it would pick up... and, if we really want to, eslint has a plugin architecture so we could add our own rules.  I will investigate and start on a PR for this.