Support for RTL languages

This is for review, and I know that some stuff can be improved, but I want to get feedback if this approach is digestable for us.

Introduces some new things to stack:
- flipcss from opera
- sass, will now create app.css and app.rtl.css
- @inclued rtl {} mixin for sass for those places where we have to do some manual tweaking, might be replaced with some more robust mixins/frameworks, but I believe it's good for now
- layout will print either app.css or app.rtl.css based on userDir 
- sends X-Vary header on userDir, so we can have separate CDN cache based on lang
Yes, but wouldn't it look better if you use what is [already there](https://github.com/Wikia/mercury/pull/452/files#diff-5db3909f0cdac37d8bc699c50c366dcdR69)? This variable would be either `ltr` or `rtl`.