Fix nested LESS selector handling

LESS selector expansion now properly recognizes `&`s.
(see http://blog.slaks.net/2013-09-29/less-css-secrets-of-the-ampersand for more detail)

I also changed the expansion and iteration code to be aware of media queries.
@mlorbetske, can you make sure that Unused CSS now reports unused selectors inside media queries?  (it should also now not warn about selectors inside mixins; making that work would be a lot more complicated)

I also introduced a unit test project, which currently only has tests for this feature.