fix(css) @import prevents parallel downloads

1. Remove import of ```normalize.css``` from ```screen.css``` to sepeed up loading of Ghost.
2. Move ```normalize.css``` to ```default.hbs```.

According to
https://github.com/stubbornella/csslint/issues/75 
http://www.stevesouders.com/blog/2009/04/09/dont-use-import/