quotes,strings fix

I noticed little bug with using strings/literals arguments from #34 

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). 

And ads a bit more consistent style in readme for this mixins. (i removed quotes, for simplicity of usage)

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 a `sprintf` stylus bif.