Fixed a spacing issue involving sizing commands and operators

This addresses issue https://github.com/Khan/KaTeX/issues/30.
This is really just turning in to special casing for all of the different interactions, which is annoying to deal with (and there are a whole lot that aren't captured here). As @spicyj points out, MathJax doesn't actually do spacing with `\Huge` correctly, and we're not entirely sure how we want to do spacing overall, so maybe we should put this on hold until we can discuss what we want exactly to happen.