First steps towards implementing CSS selectors. Exciting!
Isolate Hexp from the Sass parser
With this the basics of CSS selection are in place

So far only element, class and id selectors are supported. This should cover
80% of use cases. So you can do '#main span.foo, #main div.faz a' but you can't
do 'ul li:first' for example.

Some of these would actually be quite easy to add, but I've focused first on
the general mechanisms.
Bump version to 0.2.0, the CSS stuff calls for a release!
Update Changelog
Implement Hexp::Node#rewrite("css selector") { ... }
Tentative API expansion : add_class