2DHistogram tweaks

*Really* nice work on this new feature Flavio! The histograms look awesome :+1: 

Most of the changes are to bring the new 2DHistogram chart source code style inline with the rest of the repository. Namely

* Tabs to spaces, 2 space indent
* Comments from `//comment` to `// Comment`
* Consistent indentation

These are very picky changes, I'm a litte fussy.. :neckbeard:

I've also renamed the chart to `Histogram2D` as CSS classes can't start with numbers (I didn't know this until I tried testing it..), and tweaked how the legend is drawn.

The example histo now looks like this:

![Super cool 2D histogram](https://cloud.githubusercontent.com/assets/540853/3139897/487c5272-e8f8-11e3-9c74-58cd8264b7e3.png)