Support sorting in *.scss and *.less files

This is a clone of #88, with some differences:

- this PR is ready for merge into `dev` branch;
- tests for LESS syntax were added;
- README was updated.

![a3de86262239312ce9ce969ab810a839](https://f.cloud.github.com/assets/872004/1413382/01fdc812-3e30-11e3-97d2-b8dfc2643a50.jpg)