Clear color not an option

It’s easy enough to set it manually:

``` javascript
mathbox.world().tRenderer().setClearColorHex(0x000000, 0)
```

but since the white background is hardcoded exactly [at the position](https://github.com/unconed/MathBox.js/blob/master/src/tQuery.js#L27) where other options are applied, I’m not sure why it’s hardcoded at all. (I’m using above line of code without problem)
