quotes,strings fix

based on #34

I noticed little bug with using strings/literals arguments

this PR is fixing this behavior when we use density or orientation arguments passed as string or as literals (it converts them to one type in the end)

so we can use 

```
  +mobile(orientation: "landscape")
  +mobile(orientation: landscape)
  +mobile(orientation: 'landscape')
  +landscape(density: '2')
  +landscape(density: 2)
```
will output the same css.

But I didnt touch scale names because they can vary and for example `s` is `sprintf` stylus bif.