Make gmail-filters a client-side app

I tried out React.js (Facebook's JS library for "reactive" views) and made a static website for generating our gmail filters.

![screen shot 2014-07-22 at 6 52 50 pm](https://cloud.githubusercontent.com/assets/158675/3666551/276ecf6e-11f3-11e4-90d7-f0ed243264de.png)

I'm not too happy with how react mixes HTML with code -- going to see if there's a better way to do that.