Use constantinople to handle detecting constants

This uses [constantinople](https://github.com/ForbesLindesay/constantinople) to detect what is and isn't constant.  This has the huge advantage of allowing a lot more things to be constant, for example, you could do basic maths:

```jade
div(width= (0.3 * 1024) + 'px')
```

It uses `UglifyJS` which is the same parser that is required by `with` and minification within `transformers` so the extra dependency adds very little additional download weight.

If no issues are raised, I will merge after 2013-06-26T23:00:00Z