Start working on end-to-end LaTeX rendering performance

"end-to-end performance" meaning the full duration of a `.mathquill('latex', ...)` call

@jayferd: in Chrome on my Mac, for simple stuff like `a=6.8`, I'm getting ~1.8ms rendering the LaTeX and ~0.3ms rendering the HTML, a 6x difference

Interestingly, for more complex stuff like `\frac{d}{dx}\sqrt{x}=\frac{1}{2\sqrt{x}}`, I'm getting ~12ms / ~2ms = 6x again--it's surprisingly consistent.

You know what this says to me? This says to me that the work we did on `.jQize` and `.html` worked *too* well, we optimized HTML rendering so well it's now faster than the rest of MathQuill :D