Fix remove() bug, add css() and el(), put info in README.md

did some stuff

* `remove` wasn't removing existing text nodes from the style element, so if you called `append` and `remove` over and over, the css would repeat over and over.
* added `css` and `el` methods as a convenience for getting more info from `sassr` modules
* document what this thing does in the readme
`module.exports.getCompiledCSS`