Generates incorrect AST when media query has no expressions

I get an incorrect AST when I enter a media query without any expressions (basically anything of this format):

```
media_query: [[only | not]? <media_type> 
```

For example:

``` js
console.log(mediaquery.parse('not screen'));

/*
logs out
[ { inverse: false, type: 'all', expressions: [] } ]
*/
```

This should really be:

``` js
[ { inverse: true, type: 'screen', expressions: [] } ]
```
