Remove hardcoded URLs from css

fixes https://github.com/zeljkoprsa/waterlee-boilerplate/issues/62

tonight's MVP:
```bash
ag --sass -Q "http://waterlee.jakesharp.co/demo/skin/frontend/waterlee-boilerplate/default" -l | xargs gsed -i -E 's,http://waterlee.jakesharp.co/demo/skin/frontend/waterlee-boilerplate/default,..,g'
```

This is tested working on my install. `arrow.png` is already included in the theme even.