Load parent styles when child theme is activated

 After https://github.com/Automattic/_s/issues/635 I had another look at how the styles should be loaded when a child theme has been activated.

I spoke to @justintadlock and he mentioned that Hybrid-Core has been doing this for some time without any issues.

Responsive switched to this method soon after I wrote [an article](https://ulrich.pogson.ch/how-to-load-the-parent-styles-in-child-themes) on it last year and we have had no complaints.

The pros:
- The child theme does not require an @import and it just works with a blank style.css file.
- Not requiring the @import should bring some performance gains.