Refactor tests

I've re-factored the tests so that new test-cases can easily be created by just adding new sass- and scss-files.

On `pretest` all sass- and scss-files are passed to node-sass and compiled. The tests are then comparing if the webpack output equals the original output. This is roughly the same approach as implemented in the [less-loader](https://github.com/webpack/less-loader).

Additionally I've tweaked the README slightly.