Added support for \dfrac

Summary: The MathWidget (handwriting recognition tool) in the iOS app returns LaTeX with \dfrac
fractional expressions (rather than \frac). To avoid having to parse these in the Expression
widget or elsewhere, I added \dfrac to the KAS parser, giving it identical rules to \frac.

Test Plan: Added tests in test.html and played around in the experimenting tool.

Auditors: alex